How did Rihanna find out Chris Brown was cheating on her? She found another woman's lipstick on his knuckles.
Lipstick on Knuckles
Explanation
Explain Bear
Listen up, genius. The joke is that Chris Brown, instead of kissing another woman, was punchin' her with lipstick on his knuckles. Geddit? LOL. Since you're readin' this, you're probably not gonna be breakin' any records for smarts, but hey, at least you know you look good in that outfit!