That was a poor presentation of that trick. I've done this before and had it done to me and it has worked pretty much every time. The way I learned the trick is as such:
-You write down, "Carrot," on a piece of paper and fold it up.
-You have a friend start counting 5's and add a 5 to the end, such as 5, 55, 555, 5,555, etc.
-You have them count like that for a little while until they are really concentrating on the number 5 (usually around 55million, etc)
-Interrupt their counting and say, "Quick! Think of a vegetable!"
-More often than not they will say, "Carrot," and later not even know why they said carrot.
Only reason why I heard is cause somehow the number 5 is associated with the color orange. Don't know if that's true or total BS. But you should totally try it on someone.