If Junior needs to quickly put together a science fair project, this idea from the October 1943 issue of Popular Science will demonstrate some meaningful science with minimal preparation.  It makes use of two marbles and an old hacksaw blade, although with a bit of ingenuity, substitutions can be made to take advantage of available materials.

The two marbles are placed at the edge of a table and whacked as shown in the illustration.  Obviously, one of them flies off the table much faster and further than the other one.  But by listening to the sound of them hitting the floor simultaneously, it is clear that they are traveling downward at an identical rate.

If the teacher requires a question to be answered by the experiment, it can be something along the lines of:  “Does horizontal velocity of a projectile affect the rate of descent due to gravity?”

Junior will walk away with the blue ribbon, and the teacher will be none the wiser that he waited until the last minute.