Because Mr. Mertle asked the cops how long he had to keep The Beast chained up like a slave, they said until forever.
11 seconds sound clip from the The Sandlot (1993) movie soundboard.
You can hear this line at 00:31:20 in the Blu-ray version of the movie.
Quote context
[...]
- The Beast was too good at his guard dog job, so the police said he had to be retired.
- My grandpa, Squidman Palledorous, was police chiefback then.
- And that's where he's been for 20 years. And that's where he'll be for the rest of his life.
- Forever. Forever. Forever. Forever. Forever.
- See, man? That's why you can't go over there. Nobody ever has. Nobody ever will.
- One kid did, but nobody ever seen him again.
[...]