It's an organic compound present in tomatoes called carotine that is liposoluble but hydrophobic so the normal water and dish soap rolls right over it. In order to get rid of it pour a good amount of oil and dish soap into the Tupperware and scrub all over with a paper towel, rinse and behold the magic

it happens because you didn't buy glass containers instead, you remove it by buying glass containers in the future.

Oil/grease + tomato. Dawn.

The tomato sauce has lycopene (sp? ) in it, which is a hydrophobic molecule, making it very difficult to just wash off. You can take a stick of butter on rub it all over the inside of the container, then put a paper towel in and fill it about half the way with water. Then put the lid back on and shake the shit out it. You should get it pretty cleaned off that way.

It's facepaint, and it comes off with soap and water.

So far literally no one has said the real and correct way to remove the stains. Fill sink with some water. Add bleach, let me soak for an hour or so. Perfectly back to normal. For particularly bad stains, soak overnight. Any decent restaurant or kitchen bleaches their kitchen utensils and containers. Back when I had my food handlers card many moons ago bleaching was required.

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

Explanation of cause; basically, a part of the sauce sticks to the plastic instead of water because like attracts like.

Allrecipes.com's cleaning options.

I've been told to leave them out in direct sunlight, haven't tried yet tho.