1+1=2, metaphorically.
Give them a 1 and another 1, but do not give them 2. Ask them what they think 1 and 1 lead to.
If they are an NPC, they will freeze up and be incapable of thinking through what you just tasked them with. They will likely just chuckle and say "haha yeah whatever dude you're craaaazy" and walk off.
