An isotope is a form of an element. An element is defined by its number of protons. But each atom of an element can differ in its number of neutrons — neutrally-charged particles in the nucleus.
Some isotopes are more stable than others. Less stable isotopes may decay, or break apart, over time. For many of these unstable isotopes, scientists know how fast they decay. That is useful information. If scientists measure the amount of an isotope in something like ancient bone, they can calculate out how old that item is.
In a sentence
Scientists study isotopes from the moon to study where our planet’s tagalong came from.