Mystery #1Tier 3invisible atomic energy

What is radioactivity, the invisible thing that ticks on a Geiger counter?

Point a Geiger counter at an old rock or a lump of certain metals and it starts to click, faster and faster. Nothing looks different. Nothing smells or glows. Yet something invisible is streaming out of that lump and setting off the clicks. What on Earth is being given off, and where is it coming from?

A handheld Geiger counter with its dial and probe, the instrument that clicks when it catches the invisible bursts atoms give off — Photo: Lilly M · CC BY-SA 3.0 · via Wikimedia Commons

Your guess

Something invisible is coming out of these materials and making the counter tick. Where inside the stuff do you think it is coming from, and what might be happening there?

There's no single magic word to type, have a go in your own words, then reveal the answer to see how close you got.

Make a guess to unlock the hints and the answer.