User blog comment:FireFlows/THE ESSENTIAL GUIDE TO PERFECTING GRAMMAR/@comment-39515848-20191019150917/@comment-43788761-20191019151351

In: when a thing is physically within another thing. "The snakes are IN your mouth."

At: More general, used when "beside" or something similar would look weird. "I am AT the edge of the snake pit."

On: On top of. "The snakes are sleeping ON my mother and father."