Johnny, 

I think you are referring to the use of the preposition ‘ dans ‘ here? 

Just the way it is - in English we drink from a cup/glass/flask etc and eat off or from a plate. In French it is ‘ manger dans son assiette ‘, ‘ boire dans sa gourde/dans son verre ‘  etc 

Prepositions ! 

Laura Lawless lists this usage under the heading ‘ Provenance ‘ in 1st link below, and in the list of verbs that need ‘ dans ‘ as their following preposition before a noun in the 2nd link

https://www.lawlessfrench.com/grammar/dans/

 https://www.lawlessfrench.com/grammar/verbs-with-dans/