Sarcasm, Nature or Nurture?

Is sarcasm learned or is it in our genes?
Background: We bought Ethan a pair of robot p.j.s on clearance from J. C. Penny's; it came with a robot mask. He wore the mask in the car to Lowe's but took it off right before we got there. He asked for us to put it back on before we went inside.
Michael took him out of the car and told him to stand still so he could put the mask on him. Ethan stands there, without moving. While Michael is putting on the mask he says; "Daddy, I'm moving my eyes!" That was my laugh of the day, hope you can enjoy that with me.