For whatever reason, I thought that grass was slang for tobacco rather than marijuana.
A childhood belief about smoking

others believed things like this
- I thought for some reason that chewing tobacco was REQUIRED for Major League Baseball players.— Jeff
- I used to think that Tabasco Sauce contained tobacco, and therefore was actually "tobacco sauce."
Anon - I thought cocaine and marijuana were the same thing.Anon
- I used to think that LSD was a form of marijuana.— Shirley B.
- I used to believe that Marijuana was a disease you got from smoking.— LMH
- I used to believe that big grasshoppers would spit tobacco at you if you got too close to them.— Uni