Experiment ideas for teaching?

Quick but inspiring?

I'm looking for suggestions for instant-gratification experiments/demonstrations for elementary to high school kids. There are loads of awesome experiments kids can do that take a few days, but what about ideas that can be completed in one lesson?

Here are some ideas I've already come across:

Other ideas take a bit of preparation, but still are pretty cool to finish up in a classroom, like:

What's your favorite go-to experiment for inspiring young plant biologists?