I have to be honest, at times I'll bait transphobes online when they say that gender or sex is a biological fact; I ask them their gender and when they respond, I ask them at what age they had their sex determined scientifically in order to claim their gender identity.
It usually shuts them up quickly, although occasionally some of them will dig their heels in and I love when they do that because then we get to go through all the ways that gender is determined outside of chromosomes and of course their arguments never hold up.
One of my goals in life is to publicly humiliate every last transphobe into shutting up.