Separate
In your mouth you put a sword and a knife. Now that you have them in there, you have to make them separate from each other.
Anyone can put a sword and knife in their mouth. But not everyone can make them separate while still in your mouth.
べつ
わか
None
Why are you trying to make a sword and knife separate in your mouth? Because someone bets (べつ) you can't do it.
"I bet you can't separate the sword and knife," they say in an annoying voice. Alright, time for you to show them. Nobody bets you anything without you blindly trying to do it!