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The sexual stimulation of the penis by having it rubbed against someone else's stomach.
Visible signs of stimulation (but not required) might include twitching penis, but be careful not to confuse it with an overly excited penis that had no actual stimulation.
General twitching, indentations of the stomach as a result of pushing the penis against it, and other motion lines will also help identify tag.
A penis merely resting on the stomach should use penis on stomach.
Casual poking of the penis on the stomach should use poking with penis.
A penis being stimulated specifically by the navel should use penis to navel.