Here's an example at a bar/pub:
"Hey mate, i'd like a beer" "Bottle or glass?" "In a glass please"
You want the beer/water/juice whatever to be in a glass, so you say it like so.
It's just not natural/right to say you want it inside. Sorry for my crappy example I'm not an English teacher haha. :D
if it´s more natural to say "in" instead of "inside" I´ll use just "in". the problem with this sentence it was that without a context I thought it could be use both words. thanks!!
"inside" tends to imply an enclosed space like "inside a house". "in" can refer to anything that's capable of having things in/inside it instead of simply on/on top of them. Hope that helps :)
Here the dative form einem is used, which means that the water is already in the glass. Into a glass would be in ein Glas