Interaction-free measurement

Interaction-free measurement is a way to measure something without actually touching it. For example, imagine a room with a large box inside it. You could shine a light into the box, and by looking at the light that bounces back to you, you could tell how big the box is - and you wouldn't even have to open the box to do it. That's what interaction-free measurement is like - it's a way to measure something without disturbing it.