Tritium is a kind of hydrogen. Tritium is radioactive. Tritum has a half-life of 12.3 years. Pretend you started with 100 kilograms of tritium. After 12.3 years (one half-life), only half (50 kg) of the tritium would be left (yellow lines). After 24.6 years (two half-lives), only one-quarter (25 kg) of the original 100 kg of tritium would be left (blue lines).
Original artwork by Windows to the Universe staff (Randy Russell).