G2 shall be my favorite group! Well, for now. For a while. Or maybe for longer. It's the smallest of the exceptional Lie groups. Found 1893 by Élie Cartan. It was he who suggested to think about it in terms of rolling balls. https://en.wikipedia.org/wiki/G2_%28mathematics%29

Picture two spheres, one three times larger than the other. Imagine them rolling on another without slipping nor twisting. Rolling surfaces have their own branch of mathematics: contact geometry!

Puzzle: Starting at a pole, roll an electron around our 3 times bigger ball. After a quarter turn its orientation became the negative –s of our starting orientation s. Can we get back to s by rolling along the equator?