YA Book From 90s, Boy Who Guards Dimensions (4th Dimensional Ketchup is apparently amazing)

[Misty]

I'm looking for a book that I read probably in the 90s and it was a girl who meets a guy and finds out that he helps guard the borders between dimensions. He's not supposed to tell her about it, but he's got a crush, so he ends up taking her to visit the 2nd dimension I think it was where everything is flat, and the 4th dimension which nearly blows her mind. I remember that you could accidentally get reversed if you weren't careful - and that reversed ketchup or ketchup from the 4th dimension maybe, was so good that when she dropped a few ketchup packets people fought over them.

I remember that he gave her one rule, don't tell anyone about this, and she almost immediately told people about it and then the rest of the plot was about the chaos she caused and her and the boy trying to fix everything.

[Rosa]

The Boy Who Reversed Himself

[Kris]

Wikipedia's plot summary (with Spoilers) of The Boy Who Reversed Himself by William Sleator - Rosa's suggestion:

- https://en.wikipedia.org/wiki/The_Boy...
- There are different cover images.

Google Books has a snippet preview. You might get lucky searching for keywords:

- https://books.google.com/books?id=AmL...

Quotes from the book:

- And what other girl could offer him reversed ketchup?
"How come the label's backwards?" he said.
- Maybe four packages had been a little too much ketchup. But I refused to worry about that now.