Sai was taken aback one day when Sakura told him his mid section should be covered up, that there as nothing really special on his navel area to see anyways.
It got Sai thinking. It got him thinking really, really hard. And once Sai had wracked his brain as hard as one could wrack a brain, he came to a simple conclusion as how to make his abs more interesting to look at.
To Sakura and Naruto's utter horror, three days after the "cover your navel" comment had been made; they discovered Sai had gotten a belly button ring.
A dangling, pink jeweled belly button ring.