Fill your glasses with peach schnapps.
Slowly pour a teaspoon of Irish cream liqueur on the schnapps.
Add a squirt of grenadine.
The Irish cream will curdle and give the effect of a brain or tumor, and the grenadine looks like blood (if you add enough).
As I mentioned above, mine don't look that great, but they still look gross and taste like actual decomposing brains. So make yourself some at your next Halloween party and shoot 'em quickly. Gross.