Why do dwarf hamsters vibrate when held?

It could mean three things:

1. The hamster is excited. The hamster is being taken to somewhere else, out of his cage! New smells! New places to hide! He is vibrating with anticipation.

2. The hamster is frightened. The hamster might have been dropped before when it was being held. He might be thinking, "Put me back in the cage! PLEASE PLEASE!"

3. The hamster is cold. This rarely happens, but if it is shivering, it means it needs to sleep and "regenerate" some warmth. Don't bring short-haired hamsters in the snow or rain! It could freeze to death!