In vs. Into

To indicate movement from one place to another, use into instead of in:

walked in the room = walked within or inside the room

walked into the room = entered the room

fell in the water

fell into the water

In informal English, this rule is often ignored, and in many contexts it is not necessary to clarify one’s meaning by using into instead of in. In fact, it can sometimes sound awkward to use into in place of the more natural in.

put the receipt into the bag

put the receipt in the bag = more natural-sounding

However, be sure to follow this rule when you need to ensure the clarity of your meaning or are writing in a formal context.