In your element
Definition 1 of 1
Pro Tip 1/3Positive Feeling
This phrase often conveys a positive emotional state due to a strong personal connection with the activity. 
At the art gallery, she was clearly in her element.
Pro Tip 2/3Expertise Implication
Use it when someone not only enjoys something but also shows expertise or talent in it. 
Once he starts debating, he's in his element.
Pro Tip 3/3Natural Setting
It suggests a person is participating in an activity that feels as natural to them as their natural habitat. 
In the science lab, the students were in their element.