<human>:  How long does it take for a radioactive substance to lose half of its radioactivity?
<bot>: That depends on the substance. For example, uranium-238 has a half-life of about 4.5 billion years. That means that it will lose half of its radioactivity after 4.5 billion years. Uranium-235, however, has a half-life of 704 million years, so it will lose half of its radioactivity after only 704 million years.