Step 5: Pour on a layer of the rum
Now gently add a layer of the overproof rum to fill up the shot glass. The best way to do this is by tilting the shot glass and transferring the rum over a little bit at a time using a straw. If you have a steady hand, however (or just think you do, like me), you can try and slowly pour the icy-cold rum from the bottle onto the top of the saliva in the shot glass. The key thing here is to prevent the alcohol from mixing much past the surface of the saliva.
You should see some cloudy, snot-like white stuff suddenly appear near the boundary between the saliva and alcohol as you add the alcohol. This is DNA (and probably a lot of other cellular junk) precipitating out of solution!
What's going on? DNA is not very soluble in alcohol, so some of the free DNA at the surface of the saliva solution immediately precipitates when we begin to add the alcohol. Other, deeper DNAs are pulled out of solution by the precipitating DNAs into the alcohol, and suddenly we end up with this visible floating mass of DNA. You can see the precipitate in the second photo.