EM spectrum stuff with 2nd year today. Demos of radio, microwave, IR, UV and gamma (well...mostly beta decay from potassium chloride, but a beep or two may have been gamma...)
Everyone was most impressed by the visible bit though. Light from a projector split by a prism, this is about a metre across.
Why don't we do "proper" gamma ray demonstrations with 12 year olds? Well, there's paperwork...none of it good, it involves not exposing under-16s to strong gamma sources. Potassium chloride, on the other hand, really good every-day substance that's a little radioactive, and does give off a little gamma, so we're not totally cheating. They get to hear a Geiger counter speed up a bit...about 50% above background, which is basically like a night out in Aberdeen, granite and that.
You can buy potassium chloride as "reduced sodium salt". Most of the radioactivity from it is beta, so it can be stopped by a particularly solid sheet of paper, and at a metre or two the risk is basically zero.
But one of the decay products, Argon, then decays to gamma, which is funky.
And one in a thousand or so atoms decays via "beta minus", also known as positron emission. It produces FREAKING ANTIMATTER.
Your bones contain potassium. You make a particle of antimatter every minute or so.
@zleap Each banana adds about 1% to your daily exposure, if you live an average life in an average place.