You have to pick:

A free Would You Rather submission for anyone who knows the two pop culture references this pair is based on. :)

I suspect one would not survive driving through a puddle at high speed.

Also it could be very dangerous indoor. One accidental splash and you are sportscar in a hallway. That can't end well.

freaky friday. transformers

what cooliest112 said

Nope. A hint, then: they're both from animated TV shows. :)

Let's say that "warm" and "cold" in this case means "icewater" and "almost painfully hot", to make the circumstances a little less likely to happen by accident.

This would be problematic when someone flushes the toilet while you are in the shower.

Unless it involves me inheriting a million gazillion dollars from some jackoff member of the family I hate because they died, I'm going this way, because it sounds awesome, and it begs the question: What the hell would you call it if you artificially inseminated yourself with your own sperm?

ranma 1/2 and something else

Ranma 1/2 much?

Alright, been long enough that I'll give the answer. The gender switching scenario is from Ranma 1/2 (as guessed), the sports car one is from a much more obscure Western cartoon called Turbo Teen.