Here's an interesting physics experiment! Stack up some coins on a table, then put your finger on another coin. Now push your coin towards the stack and let go of it so that it slides and hits the stack.
If you slide your coin into the stack with enough force, it will knock the bottom coin out without knocking over the rest of the stack!

Place an ice cube into a cup of water.
Lay a piece of cotton string or yarn across the ice cube. Then sprinkle some salt onto the string.
Wait a few moments, then lift up the string with the ice cube attached to it! Salt lowers the melting temperature of ice, so the ice melts around the string, then it re-freezes around the string.

Here is an interesting optical illusion that you can make! Fold an index card (or a small piece of heavy paper) in half both ways. With a heavy marker, draw 4 lines starting at the top center of the index card, as shown in the picture.
Carefully poke a thumb tack through the center of the index card from the bottom. What will you see if you spin the index card on the thumb tack? Try it and see! Somehow the straight lines turn into a circle within a circle when you spin the card!

Here's an interesting experiment that involves gravity and balance! Hold a yardstick on your index fingers (i.e. the first finger on each hand) with your arms spread wide apart.
What will happen if you move your right hand towards your left hand? At what point will the yardstick fall? Try it! Only move your right hand (try not to move your left hand), and slowly slide your right index finger along the yardstick until it reaches your left hand. The yardstick will keep shifting so that it remains balanced!

Can you tie a knot in a string without letting go of the string? Hand someone the ends of a string as in the picture, and challenge him to tie a knot in the string without letting go of the string. He can't do it!
Here's the secret! Fold your arms, and then grab one end of the string in each hand.
Slowly unfold your arms. You tied a knot without letting go of the string!