Izaya and Shizuo were just playing a game. A simple game of 'I Bet You Can't'.
Note: I can't believe I have to write this, but please do not role play in the comments. It's a) disrespectful, considering I've asked multiple time for individuals to refrain from doing so and b) it's also just really annoying. There are other spaces for you to do that.
In addition, because apparently this wasn't enough, either, please refrain from commenting in those comment threads because you just start it up again despite my repeated requests to stop.