Galaxies: Spinning or Shivering?
Take a child's spinning top and set it on the table. It spins steadily, with all its parts moving in the same direction. Now toss a handful of beads onto the floor and blow on them: each bead will fly its own way. That's roughly how astronomers distinguish two types of large galaxies: 'spinning tops' (fast rotators) and 'beads' (slow ones with chaotic stellar motion).