What do camels living in the scorching desert have in common with polar bears wandering the freezing Arctic?
And what do they both have in common with bats?
While these four animals might look very different, they all share one common feature: fur.
And that is one of the reasons why camels, polar bears, bats and seals are all classified as mammals.
All mammals have fur.
Some mammals have a lot of hair while others, like the naked mole rat and humpback whale, have so little it's hard to see.
We have fur in the form of hair.
Fur can help mammals stay warm or it can be a camouflage, like the fur of this snow leopard, blending in with the mountainside.
And fur can give protection from predators, like the spiky hairs of a porcupine.
So even if two animals look very different, if they both have fur, they're both classified as mammals.
All mammals have fur or hair.