eli5 how does water stop radiation and not everything else outright, and how can radiation do damage to us if we are mostly made of water?

Question

eli5 how does water stop radiation and not everything else outright, and how can radiation do damage to us if we are mostly made of water?

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Answer 1

Not only does it take a few meters of water depth to stop powerful radiation (more than even the fattest person), but our cells start on the outside. Radiation that only penetrates a little bit can still do a ton of damage.

As for how water stops harmful rays in the first place I’m not 100% sure but think it has to do with its material density. The rays hit the electron clouds of the water molecules and are blocked or deflected. The same happens with lead or any other suitably dense material.