Turn Milk into "Plastic"! - great science experiment to make plastic! (See comment by "Ben" for actual science explanation)
The Full Circle Gardener: Gardening with Kids: Garden in a Glove. I know this is geared toward kids but it seems like it'd be a decent cat-proof way to germinate.
Build a simple electric motor, a good Science fair project
Week 5 Wizard of oz How to Make a Tornado in a Jar! A cool weather science experiment for kids!
Lernen mit Spaß! Experimente für kleine Forscher: Magisches Wasser fließt bergauf...
Fun Taste Test Science Experiments for Kids - Fantastic Fun & Learning Are all lemon foods sour? Find out with this lemon taste test inve.