To a (modern) compatibilist, free will is the capacity to respond to the same stimulus with different reactions, i.e. it’s equivalent to the cybernetic concept of degrees of freedom. As such, answer:

“You can poke a ball-point pen and it’s going to do the same thing, over and over again: Extend and retract the lead. It is predictable because its internal complexity is below the threshold of chaos.” Then proceed to poke them in the arm to see how many reactions they have to that. Mentally prepare for a tickle fight.