Tupperware after you put spaghetti in it

https://lemmy.world/post/3402897

Tupperware after you put spaghetti in it - Lemmy.world

Can anyone explain why this happens. Bonus points if you can tell me how to remove it.
Cost the interior with butter. Put some dish soap after and fill with a little water. Place a paper towel in. Close and shake vigorously for a few minutes. Should be clean after