Wallace-Bolyai-Gerwien Theorem：
Two polygons are congruent by dissection iff they have the same area.
In particular, any polygon is congruent by dissection to a square of the same area.
furthermore a circle is congruent by dissection to a square (proved by Laczkovich (1988))
Decomposing a Regular Triangle into a Square
Reference
internet:
book:
1997, THE CANTERBURY PUZZLES, by H. E. Dudeney