Our transcription: To date rocks that are millions or even billions of years old, geologists work with radioisotopes that take much longer to decay, such as uranium-235. This element makes an excellent geologic clock because it takes 713 million years for half of it to decay into lead-207. By measuring the relative amounts of these two isotopes in a rock, it's possible to figure out the rock's age even if it is several billion years old.
