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ApeAccount
Since: Feb, 2015
2022-02-12 22:23:01
It looks like this is A Hard Day's Knight. This link
includes the quote you mentioned.
- Candace: What do you think you're doing?Phineas: I think I'm building a horse.

So I’m trying to find a Phineas and Ferb episode where Candace asks Phineas “what do you think you’re doing?” And Phineas replies “I think I’m building (insert invention) Is this a real episode?
Edited by Gamergirl101