Backyard science experiment ideas that don't require a biology degree?

My 11yo is into science right now (finally!) and I want to feed that while I can. Don't get me wrong, I'm not a science person at all. I need experiments that:

• Don't require specialty equipment
• Can be done in the backyard or kitchen
• Don't explode or stain anything
• Are actually interesting, not just 'make a volcano'

What have you guys done that actually worked and kept your kids engaged?