Why "dans ma gourde"?

Question

Why "a bu dans ma gourde"?  Drank in my flask?  Why not "a bu de ma gourde"?

Accepted Answer

Hi Carl, 
As you will see from the following dictionary entry we use the preposition  'dans' to describe what you are drinking out of -
https://www.collinsdictionary.com/dictionary/french-english/boire-dans-un-verre
 
As opposed to -
boire un verre = to have a drink