Tim had no problem talking about that, and seems to be okay with most general subject matters (Urinary system, puberty, anything really) EXCEPT for girl's periods. In that episode he just looked at the letter, said a loud"Uh-oh," then got Cassie and Rita to just take over and talk about it- while he hid the rest of that episode!
"You have an imagination! At least, I think you do. Like, uh... that time you carved a whale-call out of a bowling pin! That was pretty creative."
"Whoa, they're climbing out of the water! I didn't know whales could do that!" (motorcycle revs) "Or ride motorcycles, for that matter!"
Tim: OH! OH! What time is it? My forehead is covered in ice! May I-I-I-I use your restroom? If I could by any type of dog, I'd-I'd be a goat. (singing) I'll be seeing you-you in all the old (Moby flicks a switch on his back as Tim slows down his singing) familiar places... (powers down)
In the episode about the Bill of Rights, Tim mentions the difference between the Sixth and Seventh Amendments. Moby has an electric guitar with him, and Tim says the Seventh Amendment applies to civil cases, "like a dispute in business, or a noise complaint." After a long beat, Tim begins to talk about the Eighth Amendment... and Moby interrupts him with the guitar.
In the episode about rainbows, Tim constantly insists that there's no pot of gold at the end of a rainbow, because there's no end of a rainbow. At the end, he and Moby see a rainbow. Moby reaches his arms all the way to the "end", stretches them back, and reveals the pot of gold he's found, causing Tim to cover the screen and say "OK, You Didn't See That!"