Mammals can live on land, water, and both.

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dolphin,whales, and porpoises are water animals. a cheetah,tiger,and lion are land animals.water and land animals have differences.water animals tend to have no feet or webbed feet  so they can swim.land animals  often have feet with claws that help move.there is a lot of differences  between land and water. 
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Photos used under Creative Commons from brownpau, philip_sheldrake