Ooh, all of it I would say. Your heart and lungs are working all day, as is your brain. Often you’ll be moving about so your muscles and skeleton are working, you’ll be eating so your digestive system is working…
One of the most beautiful things about the human body (in my opinion) is that it works so well as a complete unit.
I’m sorry I haven’t given you a specific body part, but I don’t think there is one part that works more than other parts.
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