A rectangle ABCD is drawn and both of its diagonals AC and BD are drawn in. Counting triangles of every size, how many triangles does the completed drawing contain?

The diagonals cross at a point O and split the rectangle into 4 small triangles: AOB, BOC, COD and DOA. Joining two neighbouring small triangles gives the 4 larger triangles ABC, BCD, CDA and DAB, each being half the rectangle. The total is 4 + 4 = 8 triangles.