My favorite thing to do at the mall to screw with people's heads:

Make sure you have a group of friends. Split up into groups of 1s and 2s. Have one person stand in the middle of the busiest place in the mall and just look straight up at the ceiling. Slowely have more and more of your friends happen to walk past and "see" the same thing the first person "sees". Pointing, laughing, ooohing and ahhhhing are helpful. Pulling over strangers and trying to show them the thing on the ceiling is also very effective, but not required. You probably know what happens next. Just watch as you have a large group of people getting mad when they can't see what everyone else does on the ceiling.

My friends and I thought this up one time when we were really bored at the local mall about a year ago. We made 4 tries that day, one of which was really successful. The first time we were all standing in a group looking up when one of us started laughing hysterically. This caught on and soon we were all on the ground laughing at how stupid we must have looked. We slowely became better with practice until the fourth time we probably had around 50 strangers staring at the ceiling with us!

The funniest thing, though, was that there was one little kid who asked his mom, "What is up there? I don't see anything!". The mom, who must have been in deep concentration focusing on the ceiling, ignored him and so the kid started to have a tantrum like little kids do, yelling "What is it?".