Well, Lily had! And then decided that she wished she hadn't. But Red was so much cooler than that! So very much! So he grinned boyishly and lifted his eyebrows.
"What's the difference between peanut butter and jam?" Barney waited sufficient enough time for Red to either roll her eyes and ask what or actually figure it out. When she gave up though, he took a step closer and leaned in. Then whispered the punchline in her ear.
It may or may not have been something along the lines of 'I can't peanut butter my penis in you.' Then? Then he pulled away and had to seriously keep from bursting into laughter. GREAT JOKE!