Classifying triangles by their sides and angles
We classify a bunch of triangles as either acute, right, or obtuse (classification by angles), and as either scalene, isosceles, or equilateral (classification by sides).
Then we tackle two drawing problems that concern triangles. Drawing is at the heart of what geometry is all about and it is both a hands-on activity, which students like, and also requires geometric reasoning about the attributes.
This lesson is meant for 5th grade and onward.
See also
Classify quadrilaterals — video lesson
Math Mammoth Geometry 1 — a self-teaching worktext with explanations & exercises (grades 4-5)