Muggles' Guide to Harry Potter/Magic/Reducio

Reducio is a charm that makes things small.

This spell seems to reverse the action of the engorgement charm Engorgio. The first time we see this charm used, it is used by Professor Moody to reduce a tailless whip scorpion back to its normal size after it has been used in a classroom demonstration.

Like many spells, there must be a mental component to this spell; it is necessary to control the size that you want the object to shrink to, so that size must be controlled mentally.