Classify Two-Dimensional Figures

How the lesson starts: Classify Two-Dimensional Figures

Detective Oona
Meet Detective Oona, the shape detective! Mystery shapes are hiding all over Shape City. Oona never guesses by looks. She checks clues about sides and angles. Let's crack some cases!
Right triangle clues
Triangles are sorted by their angles. A right triangle has exactly one 90° angle, like 90°, 35°, 55°. An acute triangle has all angles under 90°. An obtuse triangle has one angle over 90°, like 120°, 30°, 30°.
The parallel-sides case
Which shape always has at least one pair of parallel sides? A triangle's 3 sides all meet, so never. A pentagon? Not always. A trapezoid has a pair of parallel sides every time. Case closed: trapezoid!