They write numbers next to each one. Then you sit there and wait 20 minutes. Whatever numbers swell up and get red and itchy are the things you are allergic to. Scientific!
On Wednesday I went to an allergist because my allergies were going bonkers. They stab you in the arm and inject tiny amounts of 30 or so substances under your skin.
Then for no reason what-so-ever (ok, maybe for taking the floaty eye photo) Sarah bit me so hard that I still had this bite mark the next fucking morning. Ow!