Some say the reason most manhole covers are round is that a circle cannot fall through a smaller circular hole. Which of these other two-dimensional shapes cannot fall through a hole that is the same ...

Subscribe to Scientific American to learn and share the most exciting discoveries, innovations and ideas shaping our world today.

1. Fast and Slow Piles. This works well as a starting or closing activity. Students sort math fact cards into fast and slow piles. This visual way of tracking facts highlights which facts come ...