There are ten apples on the table. If we put these ten apples in nine drawers, no matter how we put them, we will find at least two apples in one drawer. This phenomenon is what we call the "pigeon hole principle". The pigeon hole principle roughly means: "If each drawer represents a collection, then each apple can represent an element. If there are n+ 1 or more elements in n sets, there must be at least two elements in a set. The pigeon coop principle is sometimes called the pigeon coop principle ("If there are five pigeon coops and the pigeon keeper keeps six pigeons, then when the pigeons fly back to the cages, there are at least two pigeons in one cage"). This is an important principle in combinatorial mathematics.