Secret
A grain is certain to keep a secret.
Imagine telling a piece of grain your deepest, darkest secret.
ひ
ひ
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If someone finds out your secret (not from the grain though, it will certainly keep it) then your entire face will fill with heat (ひ).
This works both ways too. If someone tells you a secret, that same heat fills you up. Now whether this heat is from embarrassment or the rush of getting to be part of a secret is up to you to figure out.