Diminishing Milk Glasses (multum in Parvo) by Mr. Magic

Four plastic cups of different sizes are displayed on the performer's table. The largest cup is found to contain milk full up to its brink. The performer pours the milk into a smaller size cup, milk does not spill over! Performer then pours the milk into a third further smaller size cup and then into a fourth further smaller size cup, milk does not spill over in either case! Thus to the audience it appears that the original quantity of milk, slowly and magically diminished to a very small quantity... this can also be done in reverse!

This looks like real magic and is VERY easy to perform.