I'm awake early and watching The Bride (of Frankenstein) with Jennifer Beals and Sting. It's a love story, not horror, but it has me thinking about Halloween.
And nothing says Halloween like fake blood. You can get directions to make fake blood with corn syrup and food coloring but the bleeding trick is very realistic and good for teaching, too.
Chemistry teachers might even use the 'bleeding trick' reaction to teach chemical equilibrium and aqueous reactions of complex ions. Bloody students would surely be memorable.
The Equilibrium Reaction (LeChatlier's Principle) from Cal State Northridge is very good. Visit their page for the teacher's guide, pictures, and links to background material. To do the Cal State demonstration, you need sodium phosphate monobasic, potassium thiocyanate, and ferric nitrate.
1. fill a petri dish with KSCN solution and a few drops of ferric nitrate solution (becomes orange)
2. add a couple ferric nitrate crystals (turns red)
3. add sodium phosphate (becomes clear again)
4. add a drop of ferric nitrate solution (red spot)
5. add a couple KSCN crystals in a different spot (orange spot)
Or students could do the experiment and make observations. After the experiment, you could write out the chemical reaction and discuss equilibrium. Then students could watch their classmates bleed!
The 'bleeding trick' only requires potassium thiocyanate and ferric nitrate. Here is a video demonstration by Bhmeta.
How To Make Blood Appear In Body Without Any Wound!!
P.S. My students tested cholesterol in fake blood samples this year. I must have a macabre fascination with hemorrhages. Boo!