I used to do this prank to my brothers all the time. The look on their face was priceless.
This works best with a styrofoam cup with a lid.
All you need is:
-One cup with lid that is in use
Step 1: Make It Fast
(I wouldn't recommending doing this to anyone that wouldn't let you touch their drinking straw! )
Wait until your prankee leaves their drink unattended.
Tear a small hole in your lemon packet
Take off the lid and straw and slide the straw into the lemon packet then replace it, submurging the packet into their beverage.
Step 2: Wait for It
When they come back, they'll take a swig and suck up a mouth full of lemon juice! They'll never see it coming!
It's happened to me before and it was quite the surprise when you're expecting to taste sweet soda.