My kids love pushpin activities - they provide great fine motor practice for the pincer grip used in writing. Today I switched things up a little by including some shapes cut out from construction paper and inviting L and E (3.5 years old) to use the pushpins to count the corners..
The kids set to work right away and knew exactly what to do. It was a great opportunity to review that triangles have three corners, rectangles have four corners, hexagons have six, etc... We also discussed the term "quadrilateral". I included all sorts of shapes including both polygons that were familiar to them and other more unusual shapes.
I intentionally included some concave polygons to get a discussion going. L didn't think the interior angle of the concave polygon counts as a corner, so we tried to come up with a definition of a 'corner' (the point where two edges meet). I'm pretty sure L was not convinced, but I'm glad I got her thinking.
This was such a fun little Montessori-inspired exercise in Geometry, that we'll definitely be repeating it again soon.