Thanks to Jessica Biel, Jimmy Fallon -- apparently a big fan of tandem bicycles -- got to have his first celebrity tandem bike ride on 'Late Night' (weeknights, 12:37AM ET on NBC).

Jessica bravely donned a helmet (helmet hair, be damned!) and tested her bicycle horn. "Don't crash us!" she said.

The pair only got as far as backstage. "You look like Pee-wee Herman!" said Jessica. Once Jimmy launched into a Pee-wee impression, yeah ... he did.