Riddles, brain puzzles and mathematical problems

[Bridge]

OK not need to hurry, take your time.

Even if you could administer 10 bottles to a single rat then you would have a problem because if the rat dies then you can't know which of these 10 bottles is the one with the poison

Spoiler below!

I know. You do 100 tests, and if any of the rats die you repeat the process taking away one bottle each time. Highly impractical and of course not possible in this situation, but it saves 891 rats.