Four different size clear tumblers are shown.
Each tumbler is twice the size of the previous one. The smallest tumbler is filled with milk.
You pour the contents of this smallest tumbler into the next size up tumbler, which is twice it's size. MAGICALLY the milk DOUBLED filling the entire tumbler!
You repeat this process with the other two tumblers, DOUBLING the milk again and again!
Perform the effect in REVERSE as well