Scientists know exactly how long it will take for half the quantity of the element to change, and this state is known as its half-life.
After another half-life has passed, the element will have decayed to a quarter of its original amount.
The Age of Dinosaurs was so many millions of years ago that it is very difficult to date exactly.
Scientists use two kinds of dating techniques to work out the age of rocks and fossils. This considers the positions of the different rocks in sequence (in relation to each other) and the different types of fossil that are found in them.
We'll explore both relative and numerical dating on our quest to understand the process of geological dating.