That's when you came up with the idea for the flux capacitor, which is what makes time travel possible.
9 seconds sound clip from the Back to the Future movie soundboard.
You can hear this line at 00:50:47 in the Blu-ray version of the movie.
Quote context
[...]
- I've had enough practical jokes for one evening.
- The bruise-the bruise on your head, I know how that happened. You told me the whole story.
- There's something wrong with the starter, so I hid it here.
- After I fell off my toilet, I drew this.
- It works! It works! I finally invent something that works!
[...]