Paris has two million inhabitants. A human being has, at most, 600000 hairs on their head. What is the largest number of Parisians that we can hope to find who have exactly the same number of hairs on their head?
Here, the drawers are the numbers of hairs and the socks will be the Parisians. We then have 2000000 socks for 600001 drawers (we don't forget the bald Parisians): the pigeonhole principle assures us that it is possible to find $\left\lceil\frac{2000000}{600001}\right\rceil=4$ Parisians who have exactly the same number of hairs.