Earlier this year, a New Jersey high school student brought a broken piece of an antique dinner plate to class. The student wanted to use it to show his classmates his new Geiger counter. This device measures highenergy particles and waves called radiation. Using the Geiger counter, he demonstrated that the piece of plate was radioactive!

The glaze that was used to give the ceramic plate its hard outer coating contained the element uranium (U). Uranium decays, or breaks down over time, releasing radiation. Exposure to highly radioactive materials can be dangerous to people. To be safe, the whole school was evacuated.

That prompted scientists to write a letter to school officials explaining that the student’s plate hadn’t posed a real threat. That’s because uranium is not highly radioactive. Certain school items, like some glow-in-the-dark exit signs, also emit low levels of radiation, says Jake Hecla, a Ph.D. student at the University of California, Berkeley, who helped write the letter.