Why is the sky blue?
What is lightning?
What are the states of matter?
Does hot water freeze faster than cold?
Why does a can of Coke sink while a can of Diet Coke floats?
Why do you close your eyes when you sneeze?
Why do mosquitoes bite me more than my friends?
Why do your feet stink?
How does gravity work?
How fast does the space shuttle move in orbit?
What is escape velocity?
What would happen if you turned on your headlights while moving at the speed of light?
Isaac Newton figured out how gravity works, and even wrote a formula to calculate gravitational attraction between two objects from just four numbers: the masses of the objects ("M1" and "M2"), the distance between their centers of mass ("R"), and a constant (conveniently called "G").
If you can stand to look at the formula, you'll see that closer and more massive objects are attracted by gravity more than less massive, more distant objects. Incidentally, for those who (like certain Why Filers) have been bothered more by gravity lately, you'll be happy to know Earth feels your pain. To put that another way, you exert the same pull on Earth that it does on you.
But it's a lot easier explaining how gravity acts than why it acts. Gravity is thought to produces waves -- somewhat like the waves that bring us sunlight, Howard Stern and MTV -- but they have proven fiendishly hard to find.
One large project, the Laser Interferometer Gravitational-Wave Observatory, is setting up two huge, expensive instruments to try to detect gravitational waves.
They're not trying to find out why gravity seems so much stronger at the end of a long day. Instead, they're looking for the gravitational signature of gigantic collisions between black holes, rapidly spinning neutron stars, and other astronomical exotica.
