A triangle is a flat shape with three straight sides and three corners (called vertices). The three sides connect to form a closed figure, and no matter what kind of triangle you draw, those three angles inside will always add up to exactly 180 degrees.

Triangles come in different varieties. An equilateral triangle has three equal sides and three equal angles. An isosceles triangle has two equal sides. A scalene triangle has three different side lengths. You can also describe triangles by their angles: a right triangle has one 90-degree angle (like the corner of a square), while an acute triangle has all angles less than 90 degrees, and an obtuse triangle has one angle greater than 90 degrees.

Triangles are remarkably strong shapes. Engineers use triangular supports in bridges, towers, and buildings because triangles don't collapse easily when weight pushes down on them. The Eiffel Tower, for instance, is essentially a network of thousands of triangles stacked together.

You'll find triangles everywhere: in yield signs, slices of pizza, roof trusses, the sails of boats, and musical instruments. In music, a triangle is also a percussion instrument: a metal rod bent into a triangular shape that makes a clear, ringing sound when struck with a small metal beater.