system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:If dolphins and whales are mammals who don't have gills and need to frequently come to the water surface then why can't they survive in land?
gpt
Imagine an astronaut having spent one year in space. Despite all the exercise to maintain his strength in a zero-G environment, his heart will have shrunk by two third, his bone mass will have reduced dramatically and its muscles will have followed the same path. Back on Earth, the astronaut will be carried out of the capsule straight onto a wheelchair, and it will take weeks before he can get back on his feet.

Life in the sea is like living in zero-g. But now, consider these mammals having spent their WHOLE LIFE in that environment, following millions of years of evolution inside that environment, without having trained their body to support their weight on land... You can now picture the disaster their body will face once on land.