Ha, I remember this from an old Open University TV lecture. They suggested looking at the spin of a particular particle. So eventually the alien arrives and it

Ha, I remember this from an old Open University TV lecture. They suggested looking at the spin of a particular particle. So eventually the alien arrives and it goes to shake hands/tentacles, but it’s not holding out the right one as it has been taught is customary … BOOM annihilation Ah, the alien was made from antimatter, where the spin is different. Oops.