alright this is what you do when your victim walks away (make sure you have enough time) pour out there soda and then put water back into there container the add a shit load of soy suace (enough to make it dark looking)to there drink. Now when your victim returns they will take a swig and it will taste so bad they'll have to wash out there mouth!This work best on people who always ask for your soda (you know,those pesistant people who ask if you your going to drink it) or always ask for a sip and take huge gulps

P.S. add a little bit of real soda for the carbanation effect.