ins = in + das
use "ins" when you use the preposition "in" together with the specific direct article "das".
Ich gehe INS Haus = Ich gehe in das Haus BUT NOT: Ich gehe ins Bar - BECAUSE: Ich gehe in DIE Bar.
This rule applies to several prepositions like:
durchs = durch + das
aufs = auf + das
übers = über + das
unters = unter + das
Hope I could help you :)
ins = in das
This might help you, just think of the bed not only as the bed, but the pillow and the blanket are part of the bed. Now you are entering the bed, because you will go under the planket.