Here's a little scenario I love to watch people try and wrap their brains around. If they get it, and it usually takes a little extra expanation, then they are in the right frame of mind to get other aspects of ballistics.

You have two bullets, one that you are going to drop by hand and one that will be fired from a gun. The gun's barrel is perfectly level with a perfectly level floor that extends for miles. If you drop your bullet from the same height of the gun barrel at the exact moment the fired bullet leaves the barrel then both bullets will hit the floor at the same time. (See attatched picture)

It's really cool seeing the "lights come on" when someone gets it.