Simplify the following text for a Grade 8 native English-speaking person.
Original: In a moist environment , plutonium forms hydrides on its surface , which are pyrophoric and may ignite in air at room temperature . Plutonium expands up to 70 % in volume as it oxidizes and thus may break its container .
Simplified: It forms oxides and hydrides in moist air , causing the plutonium sample to expand by up to 70 % .