A pentagon is any flat shape with exactly five straight sides and five corners (called vertices).

You see pentagons in everyday life more than you might think. Home plate on a baseball diamond is a pentagon. Some street signs are pentagons. Artists and architects use pentagons in their designs because the five-sided shape creates interesting patterns and symmetry.

The most famous pentagon in the world is the Pentagon, the headquarters of the United States Department of Defense near Washington, D.C. This massive building has five sides, five floors above ground, and five ring corridors on each floor. It's one of the world's largest office buildings, and when people write it with a capital P, they're referring specifically to this building or the military leadership that works there.

Mathematicians find pentagons fascinating because you can't tile a floor using only regular pentagons (ones with all equal sides and angles) without leaving gaps. However, some irregular pentagons can tile perfectly, and discovering new types of pentagon tilings is still an active area of mathematical research. In 2015, a team discovered the 15th type of pentagon that can tile a plane.