Solution
= Solution
The <pigeonhole principle> states that if more than $r$ objects are placed in $r$ boxes, with $r\geq1$, at least one box contains two or more objects. Equivalently, a <function> from a finite <set> of more than $r$ elements to a <set> of $r$ elements cannot be injective: if every box held at most one object, there would be at most $r$ objects altogether.