alternatively you just banged on your leg and while the impact wasn't strong enough to penetrate through the cloth, it was enough to rupture your skin. this can also be done if something skids right off - especially true in those cases. it's basically the same concept as a bruise.
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The King Wang.
Listen up Urinal Cake. I already have something that tells me if I'm too drunk when I pee on it: My friends. - Colbert.