Deciding to take the old man's suggestion, you try to swap your height with a tall black dude nearby. He looked to be a bit over 6', as opposed to your more average 5'8". You hold the necklace, and think about swapping.
Suddenly, your point of view shoots up! You get whiplash for a split second, and stand there with your head on your forehead, feeling incredibly dizzy. The feeling subsides, and you look down to see that the ground looks much farther away than it did just a few minutes earlier. You look over to the black guy, who's just talking on his phone like nothing happened.
The necklace worked! You grew taller, and no one noticed! You do a quick physical inspection of yourself, and you can't find any evidence that you swapped with someone other than the black guy. You chuckle. Clearly the old dude put it in the bin by accident and didn't want you to buy it. You swap back with the black guy. This was going to be fun-you could swap anything with anyone, with no consequences!
Unbeknownst to you, you had swapped with two strangers. However, the things you swapped weren't immediately noticeable. You remained ignorant of them until you got to...