Genesis 4:18

And unto Enoch was born Irad: and Irad begat Mehujael: and Mehujael begat Methusael: and Methusael begat Lamech.

1. Enoch, the son of Cain, had a child named Irad.
2. Irad, the grandchild of Cain, had a child named Mehujael.
3. Mehujael, a great-grandchild of Cain, had a child named Methusael.
4. Methusael, a 2nd great-grandchild of Cain, had a child named Lamech.
5. The people kept track of their genealogy.