A Geiger counter is a handheld device that detects and measures radiation, making clicking sounds that speed up when radiation levels increase. Scientists, safety workers, and nuclear technicians use Geiger counters to check if an area is safe or if materials are radioactive.

The device works through a special tube filled with gas. When invisible radiation particles pass through the tube, they cause tiny electrical pulses that the counter converts into audible clicks. The faster the clicks, the more radiation is present. This simple feedback system lets users immediately know whether they're in danger, similar to how a smoke detector beeps when it senses smoke.

Geiger counters became famous after nuclear accidents, when workers used them to map contaminated areas and find radioactive materials. They're also used in hospitals to handle medical isotopes safely, in labs studying radioactive rocks and minerals, and even by some hikers exploring old uranium mines.

When people in movies or books describe a Geiger counter “going crazy” with rapid clicking, it means dangerous levels of radiation have been detected.

While most people will never need to use one, Geiger counters represent an important principle: making invisible dangers visible and measurable so we can protect ourselves and others.