I haven´t had any problems with "in or into" as well as "in to" until now.
I know how to differentiate the three, but I can´t explain the following examples:
"You can put the milk in/into the fridge!"
"Do not let your skin come in/into contact with the acid!"
This is said to have the same meaning, therefore, one can use both. But why is it the same in meaning, really?
You can´t say "Come into" when asking someone to enter a room, can you?