Skin
If you look in a tiger's stomach you'll find a bunch of skin. It's really gross but what else did you think you'd find? Everyone knows tigers can't digest the skin of the things it eats.
Imagine yourself opening a tigers mouth and looking all the way down into its stomach and seeing a bunch of skin. Why would you imagine that? Eww.
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はだ
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The only person who would try to take skin from a tiger's stomach is a fool (ふ). You'll probably get your head bitten off looking for skin that way.
Imagine a fool trying to pull skin out of a tiger's stomach. You can imagine them getting their skin added to the pile too if you want.