This is a tricky puzzle question I thought up some time ago, but I figured I would blog it.
As people who study physics know, the acceleration a falling body undergoes if dropped (in a vacuum) at the surface of the earth is known as "g", or 9.8 meters per second per second.
This is so close to 10 that most students and people doing back of envelope calculations often use 10 as the value of a "g". It's easy. Fall for one second and you're going 10 meters/second.