We recently came across this little trick at a convention in Atlanta. Basically, if you place a billiard or snooker ball on top of another billiard ball, the top one will fall off and roll away at some speed. However if you place a light weight drinks can (the ones made of aluminium are the lightest, and show the trick best of all) over the two billiard balls - having cut one end off the tin - and then place a third billiard ball on top in order to hold everything in position, you can then pose the question 'what will happen when I remove the billiard ball sitting on top of the tin?'
What is interesting is that two things happen. Firstly, the tin shoots off to one side with a considerable speed, as the top of the two billiard balls inside the tin tries of fall off the bottom one. However in giving the tin that sideways force, the top billiard ball loses much of the momentum that it would otherwise have had, and ends up rolling away very slowly indeed. Nearly all of its sideways momentum has been transferred to the tin.
Finally Tim demonstrates a little bit of fun with tins that seem to slide over the table under their own momentum. Actually, he has popped a very large ball bearing - 2 inches in diameter - under the tin, and it is the ball bearing that allows the tin to keep sliding across the table.