My favorite "trick" (which isn't magic, but SCIENCE!!) at the dinner table requires two forks (they need to be the same kind of fork), two toothpicks, and a salt or pepper shaker.
Stick one toothpick into a hole in the shaker. Connect the forks by placing the teeth on the forks between each other. A bit hard to describe. Take the other toothpick and place it through the teeth of the forks. If you did it right, the forks should balance on the toothpick if you hold the end. Now, the part that's just too weird to believe -- take the end not connected to the forks and balance it on the very tip of the toothpick in the saltshaker. If you did it all right (I know, my directions were vague and hard to follow... it's a bit hard to describe), the system should perfectly balance on the tip of the toothpick in the saltshaker because its center of mass, remarkably, is DIRECTLY over the tip. The natural reation to this is to want to spin the system. It usually gets about halfway around before it falls. I can't figure out why either of these things are true.
