Get them to play with toys that demonstrate scientific principles. It's amazing how much they absorb. Then you will have a solid basis from which to say "How does this work? What do these have in common?"

I found this link some time ago here on HN:

http://www.arvindguptatoys.com/toys.html

Pick two or three, make the toys, play, then ask "What did you see?" and "Why is it so?"

I don't know your experience in teaching, but your willingness and enthusiasm are fantastic.

Good Luck!