If you put your glass bowls, such as stackable Pyrex bowls, into the cupboard while still hot from the dishwasher or sink, you may discover they are stuck together when you next go to use them. But how can you solve the problem of stuck bowls? Actually, it is simply when you think about it, and it also works for glasses or measuring cups that are also stuck together.
Fill your sink with hot water high enough to reach close to the top of the bottom bowl. Then fill the inner bowl with cold water. Wait a minute or two, and the heat should cause the bottom bowl to expand slightly while the cold water will cause the top bowl to contract slightly. You should be able to pull them apart easily.
To prevent the bowls from getting stuck in the future, you can place a piece of parchment paper or a coffee filter between them. There are also special inserts you can buy that are meant to place between dishes, however, they can be expensive.