Some mammals use twigs or stones as tools. The sea otter floats on its back in the water with a large stone balanced on its stomach. It smashes shellfish against the stone to open them up to eat. The otter keeps its face turned away to avoid being hit by sharp pieces of broken shell. One otter was seen to open 54 mussels in 86 minutes. It smashed them against a stone over 2,000 times.