Why does my travel mug leak in my bag?

I have a travel mug/tumbler from starbucks that I carry with me for work. Usually it doesn t leak at all as long as the lid is on tight but almost all the time whenever I have it in my bag and I m commuting it ends up leaking all over and making everything in my bag wet. The lid screws on and it has a plastic piece... show more I have a travel mug/tumbler from starbucks that I carry with me for work. Usually it doesn t leak at all as long as the lid is on tight but almost all the time whenever I have it in my bag and I m commuting it ends up leaking all over and making everything in my bag wet. The lid screws on and it has a plastic piece I lift to drink from it.
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