My daughter knows that a stop sign is an octagon. But to help her identify the difference between an octagon and other shapes, we made a variety of stop signs, some correct and some incorrect shapes, including tricky ones like a hexagon, which is ALMOST a stop sign. It gets her thinking mathematically in an everyday way, and also helps her to learn and identify other traffic signs and signals and their meanings. The library has some great books on this (try Signs in our World) and a short walk in any town or city is a fun way to apply what you're learning. My younger son (who loves construction and trucks) is loving these lessons as well.
Counting the sides: the kids made STOP signs that were circles, hexagons, pentagons and the correct octagon shape. Working it into a hands-on craft for their toy cars took it off the page and into a concrete lesson. Identifying what is not an octagon was just as valuable as learning what one is. I like the reminder that sometimes, with anything, the way to approach learning the correct way is through examples of the incorrect.