The riddle
What Do You Call a Destroyed Angle?
Wrecked angle, rectangle
Break "wrecked angle" after its first word and the sounds are r-e-k-t and then a-ng-g-l. Wiktionary gives rectangle as /ˈrɛktæŋɡ(ə)l/, so the two phrases are made of the same sounds. Only the stress and the pause move. The word destroyed stands in for wrecked. People also search the joke as "what do you call a crushed angle", and it is told with broken in the same place, and the answer is still a rectangle.
What the word really means
The real root is not wreck. Wiktionary traces rectangle through Middle French to Late Latin rectangulum, meaning right angle, built from rectus, right, and angulus, angle. So the front of the word means right, the same root that gives English rectify, to make right. The joke uses an accidental sound, which is how most puns work.
The geometry, checked
A rectangle is a four-sided shape with opposite sides parallel and four right angles. Each right angle measures 90 degrees, and four of them add up to 360. That is the same total as for any four-sided shape whose sides do not cross, because such a shape can be cut into two triangles of 180 degrees each. A square is a rectangle with all four sides equal, so a square is a rectangle too.
Another angle on angles
A geometry joke built on a different word is why did the obtuse angle go to the beach. A classroom can collect shape puns and sort them by which trick they use.