You learned this reading when you learned 転がる, but here's a mnemonic anyway just in case:
When you try to roll something, the outside bits may be moving, but its core (ころ) stays still. Picture yourself making the earth roll across the floor. The outside parts where the people live are spinning round and round, but the earth's core is in exactly the same place.
I rolled a can of juice.
The baby likes pushing the doll to turn it over.
Dung beetles are called “poop rollers” because they roll poop.