Difficult

Man walks over, and man swims under. In times of war, it can be burned asunder. What is it?

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What has roots that nobody sees, and is taller than trees. Up, up it goes, and yet it never grows.

Who eats a lot of iron without getting sick?

Though not a plant, has leaves. Though not a beast, has spine. Though many wouldn't need this thing, It's more valuable than wine.