1:Why is the moon sometimes out during the day?
The moon is just as likely to be visible during the day as it is at night — it orbits Earth independently of the sun. When its orbit brings it to your part of the sky during daylight hours, it is illuminated by the sun, and we can see it.

2:Why is the sky blue?
The light coming from the sun is made of many colors; light travels as a wave, and each color has a unique wavelength. Violet and blue light has shorter wavelengths, while red light has a longer wavelength, and the other colors have wavelengths in between.

When the different colors of light pass through the atmosphere, they run into molecules, water droplets and bits of dust. Because all these particles are closer in size to shorter wavelengths of light, they tend to scatter violet and blue light much more than red, and so they send rays of violet and blue ricocheting toward the ground — and your eyes. More violet light actually gets scattered by atmospheric particles than blue light, but your eyes are more sensitive to blue, so the sky appears blue.

Sunsets are orange-red because in the evening, with the sun low on the horizon, sunlight must pass through more atmosphere to get to your eyes, and only the red light can make it all the way through. The shorter wavelengths have all been scattered toward the ground in the part of Earth where it is still daytime. [The Physics of Rainbows, and Other Everyday Things]

3:Will we ever discover aliens?
No one knows how rare alien life is in the universe, so there's no telling whether humanity will ever manage to discover it. However, scientists at the SETI Institute in California, who are engaged in the search for extraterrestrial intelligence, are hopeful that they'll detect alien signals within the next 20 years. The scientists scan the night sky looking for unnatural radio or light beams — ones that could only emanate from an intelligent civilization.

Their 20-year estimate is based on the rapid pace with which astronomers are discovering planets beyond our solar system, including planets that seem suitable for life; it is also based on the assumption that, if there are intelligent beings out there, they, too, will seek contact with others, and will make their presence known by sending signals into space.

4:How much does the Earth weigh?
The first approach to answering this question is to get technical about it. Because the Earth is in free fall around the sun, it actually weighs nothing. The same goes for astronauts in orbit; because they are technically falling around the Earth — and if they stood on a scale, it, too, would be falling — the scale would read zero.

Alternatively, you could discuss the Earth's mass — a property that is independent of where an object is in the universe, or what it is doing. Earth has a mass of 5.97 × 10^24 kilograms — the equivalent of one hundred million billion Titanics. [What If Everyone on Earth Jumped at Once?]

5:How do airplanes stay up?
To overcome the forces of drag and gravity, an airplane must generate two forces of its own: thrust and lift.

Thrust is the force that propels an airplane forward on the runway. By Newton's third law — every action has an equal and opposite reaction — the plane's engine generates forward thrust by spewing fuel backwards. Next, as the plane hurtles down the runway, each of its wings slices the air into two streams, one that flows above it and the other, below. The wings are shaped in such a way that the air flowing over them is ultimately deflected downward, and, again because of Newton's third law, the downward motion of the air causes an equal and opposite upward motion of the plane. This is lift. [Do Planes Get Struck by Lightning?]

Every airplane has a specific takeoff speed — the point at which lift overcomes gravity. That critical speed changes based on how much a particular plane weighs. The plane's engine, meanwhile, has to work to provide enough thrust to overcome drag — friction with the air.

6:Why is the sky blue?
The sky is blue because the Sun produces white light which is made up of all the colours of the rainbow.

But a clear, cloudless day-time sky is blue because gas molecules and particles in the air scatter blue light (which travels as short, small waves) from the Sun more easily than they do red light (which travels as longer, larger waves).

As the Sun sets low on the horizon though, the light has even more of the atmosphere to pass through before reaching our eyes, so more blue light is scattered, allowing more of the red and orange light to pass straight though to our eyes.

7:What is a rainbow?
A rainbow is made from light and water – with help from the Sun. Sunlight is refracted (bent), reflected (bounced off the inside) and then refracted again through droplets of rain, splitting it into the colours that we see (red, orange, yellow, green, blue, indigo and violet).

However, you have to be in the right place at the right time! To see one, you have to be between the Sun and the rain (with your back to the Sun) AND for all of these to be aligned so that the Sun, your eyes and the centre of the arc of the rainbow are in a straight line!

As this tends to mean we can only see a rainbow in the morning or late afternoon, you can work out where to look if conditions are right: a morning rainbow appears when the Sun shines in the East, and the rain falls in the West, and an afternoon rainbow appears when the Sun shines in the West, and the rain falls in the East.

8:What causes the tides?
Tides (the rising and falling of the oceans and seas) are caused by the gravitational pull of the Moon on the Earth. To a lesser extent, the Sun’s gravitational pull also impacts on the tides.

The Moon’s gravity ‘pulls’ at the Earth’s seas as it orbits the planet, causing buldges on both the side of the Earth the Moon is on and the opposite side (because the Moon is pulling the Earth away from the water on that side).

If you’re at the coast and the Moon is overhead, you’ll experience a high tide. This’ll also be true if you’re on the opposite side of the Earth.

Therefore, there are two high tides and two low tides at every coastal location each day.


9:Why are bubbles round?
There is surface tension on the bubble’s skin that keeps it pulled in as tightly as it can. As air molecules trapped inside the bubble move around in all directions, its skin tends to form a sphere, being the shape with the least amount of surface area compared to the volume of air trapped inside.

10:What are clouds made of?
Clouds are small droplets of water vapour (water gas) or ice crystals that are light enough to float on air. Water from the oceans and ground evaporates as it heats up and rises into the air, where it condenses on tiny dust particles. As they cool, the droplets grow bigger and become heavier they start to fall as rain, sleet or snow.

11:How do plants grow?
Plants need several things to grow. They make their own food (in the form of sugars) in a process called photosynthesis. Photosynthesis happens in the presence of sunlight, when water taken in through the plants roots is combined chemically with carbon dioxide gas taken in through small pores called stomata on the plants leaves.

But, they also need to be within a suitable temperature range and have access to the right minerals from the soil.

12:Why do bees buzz?
It has now been discovered that it is not the wing beat that actually causes the buzzing noise, but instead it is all part of the complex flight system found in bees and flies known as the ‘click mechanism’. This mechanism is what allows the bees to flap their wings fast enough to provide enough thrust and lift to fly their relatively heavy bodies.

The buzzing noise is created as the bees thorax clicks in and out of place at high speed, which enables them to not only flap their wings, but also to twist and turn them at an angle as they beat up and down, which reduces drag and allows the bee to beat their wings even faster. This means that they can get as many two beats or more for every tiny muscular contraction, which equates to as many as 200 beats per second. The click of the exoskeleton resonates in the thoracic cavity and creates that characteristic buzzing sound.

Scientist also think some bees, such as the larger bumble bees, also use this buzzing vibration to help shake pollen free from flowers, and this may explain why some bees still buzz loudly when they are on flowers, especially plants such as tomatoes and honey suckle as these have tubular anthers
which are more difficult for larger bees to reach the pollen inside.

13:How many stars are in the sky?
Answer: Look-up at night at a clear, moonless sky from a city and you’ll be lucky to see a few hundred stars. Get away from the light-polluted sky though and you’ll see a couple of thousand.

Like our own local star, the Sun, each one of these twinkling points of light is a giant nuclear reaction. In our home galaxy, the Milky Way, it’s estimated there are somewhere in the region of 100 to 200 billion of these burning balls of gas.

However, the Universe appears to contain almost a similar number of galaxies as our galaxy contains stars. That means there could be 1,000,000,000,000,000,000,000,000 stars in total, if you assume all galaxies are the same size (they’re not – some are much bigger, some much smaller). But, even that number doesn’t take into account the fact that stars are constantly dying and new ones being born… so don’t expect to ever get an exact number!

14:Why does wood float, but stones sink?
The answer to why wood floats and stones sink lies in density. Density is a measure of how closely packed the particles in an object of a certain volume are.

Effectively, if you were to fill a space with water that was the same volume as one filled with wood, the space filled with wood would be lighter as it contains less mass.

Those things that are less dense than water will float, whilst those that are more dense sink.