Do you have any idea how difficult it is to get a head into a jar? I mean, sure: you can try to squeeze it into a bulk mayo jar with a wide mouth. But believe me, that skull is going to get in the way. And you can't just go around beheading people until you find a head that's the right size to actually fit: that would be totally uncool, and it would keep you from getting invited to the better Halloween parties.
...Or so I've heard.
But faking it? Faking it is easy. Check out these Head in a Jar instructions from Mad Haus Creative.
All you need is a large jar -- the sort that looks like it might be large enough to hold a head -- and a photo of a corpse-like face. In this instance, the photo is a texture map created for 3-D animators. Print it out, pop it into the jar, push it around until it looks right (or, more to the point, utterly sick and wrong), maybe add some hair... and bam, you're in the garage scene from Silence of the Lambs.
Deeply squicky, yet almost completely effortless! That's my kind of scary.