Tupperware not air-tight or liquid-tight?

Question

I received an e-mail from tupperwares that their some of their products are not meant for air-tight or liquid-tight like the square-A-way or square round containers.

Why aren't they tight-proof?

Answer 1

It would be due to the way the plastic is made during the molding process. If you'll look at the rim & lids you'll see the edges are almost straight where the water tight container has a rounded edge that the lid seal goes into a channel on the bowl.

Answer 2

The 'burp lids' are the only ones with the tight seals; they DO work.