This is my go-to. "Hey, kids - let's learn how to cut glass tubes and produce gases like Hydrogen and Chlorine!" Most parents recoil.

But it's not really that dangerous in a well-ventilated, adult-supervised setting. The Hydrogen will make a nice "pop", and the chlorine might rust a few screws.

The important thing is to make the kids feel like they are learning about dangerous, forbidden knowledge. That'll help them stay safe, and encourage them to learn more.