03/16/2012 11:07 pm ET Updated May 16, 2012

Alan Alda's Flame Challenge and Kids' Five Most Popular Science Questions

The actor Alan Alda challenges you to explain to an 11-year-old what a flame is.

The winning entry will be announced at the World Science Festival in New York in June. According to researchers for the Big Bang United Kingdom Young Scientists & Engineers Fair, the top questions asked by children from 5 to 16 years old are:

1. Why is the Moon sometimes out in the day?
2. Why is the sky blue?
3. Will we ever discover aliens?
4. How much does the earth weigh?
5. How do aeroplanes stay up?

To the first question on why the Moon is sometimes out in the day, the Big Bang Fair suggests this answer:

The Moon is lit by the Sun. So when its orbit brings it to the right place, we can see it.

Life's Little Mysteries suggests this answer:

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.

I think that those are both poor answers. The real question is, Why is the Moon sometimes out in the day when none of the stars are out? And my answer is:

The Moon is so big and bright that we can see it even during the daytime, when the Sun's light makes the sky too bright for us to see the other stars and planets. At sunset, you can watch all the other stars and planets gradually appear as the sky darkens.

Life's Little Mysteries has an excellent answer to the third question, "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, scientist at the SETI Institute in California, who are engaged in the search for extraterrestrial intelligence, are hopeful that they'll detect alien signals with in the next 20 years. The scientist 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 week contact with others, and will make their presence known by sending signals into space.

I was hopeful of contact with aliens until I read a mathematical argument by geometer Ken Brakke of Susquehanna University. According to his mathematical model, even if the galaxy potentially has millions of space-faring civilizations, the first such civilization probably gets about a 100-million-year head start on the second. If there was another before us they would already be throughout the galaxy, so it looks like were the first, and the galaxy is ours to colonize alone. Brakke just wrote to me: "Now if only all the UFO hunters and ancient alien enthusiasts read your blog, the world will be a saner place."

Another interesting question is "How much does the earth weigh?" The Big Bang Fair gives this confusing technical answer:

The earth technically weighs nothing, because it's falling around the Sun.

Let's try this technical tack. What is weight? It's the gravitational pull of the Earth on an object. Does the Earth pull on itself? Yes, different parts are pulling on each other. But the net force is zero, because if portion A is pulling portion B in one direction, B is pulling A back in the opposite direction.

But the real question is, how much stuff or "mass" is there to the earth. Here the answer is 6000000000000000000000000 kilograms, roughly comparable to 13000000000000000000000000 pounds or, as Life's Little Mysteries puts it, one hundred million billion Titanics.

Fianlly, here's my answer to the question, what is a flame?

A flame is hot gas on fire, emitted from other burning material. It gives off heat and light as it consumes oxygen from the air.

Can you do better? Contest entries are due by April 2.