1) Go to McDonalds and buy a burger with french fries (make sure they put it in one of those easily recognizable 'McDonald's bags'
2) go to the metro (subway)
3) give the bag to your friend and tell him to come into go to the next station and then enter the next train (where you will be) and hand you the bag.
4) get into the train after your friend's
5) Now - most trains have some sort of intercom (for emergency purposes) with the engineer, so you want to go up to this intercom box, make it look like you are pressing the button and very loudly say (so everyone can hear) "Yeah, hi, I would like a big mac and ummm..medium fries please, thanks!"
At this point people start looking at you 'funny'.
6) At the next station your friend walks in, hands you the bag, and you hand him some money, he then leaves the train at that same station.
7) start eating the big mac or fries to show the people around you that its real

The look on the people's faces is amazing! Especially the older people (they start yelling at you immediately). If you've got nothing to do...try it, and post the results on here so then we can compare (I wanna see the different reactions in the U.K. and U.S. since I did this in Moscow!)
Well, have fun!
Vic