system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do crabs work?

I'm baffled. I'm laying here hungover thinking about eating crab legs when I realize: I don't understand crabs. 

They're made of three parts: shell, meat, and that spindly tendon you pull out. How does that make for movement? Am I missing something here?

Please educate me on crab movement and physiology and just give me interesting crab facts in general. I need to know.
gpt
The meat inside their shells is essentially their muscle. It's used to contract and extend their appendages similar to the way you do for your arms and legs.

Their organs are contained within the shell (along with some more meat).