Then, one night, I looked upon the words under the cage door and understood them.

11 seconds sound clip from the The Secret of NIMH (1982) movie soundboard.

You can hear this line at 00:52:43.209 in the Blu-ray version of the movie.

Quote context

[...]

- We were captured, put in cages, and sent to a place called NIMH.

- There were many animals there in cages. They were put through the most unspeakable tortures to satisfy some scientific curiosity.

- Often, at night, I would hear them crying out in anguish.

- 20 rats and 11 mice were given injections.

- Our world began changing.

- Then, one night, I looked upon the words under the cage door and understood them.

- We had become intelligent. We could read.

- The miracle was kept secret from the scientists, and in the quiet of the night, we escaped through the ventilation system.

- The mice were blown away, sucked down dark air shafts to their deaths. All, except two: Jonathan and Mr. Ages.

- We were trapped by a locked door on the roof.

[...]