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Lactose bridge intolerance

Wow, this is kind of fiddly. The intended solution is almost certainly far too obscure; unfortunately there's not a great deal I can do about this but to offer some hints:

1) If you see something weird happen in the first room, you're on the right track.

2) You need to achieve the same thing in the third room as you did in the second.

3) No room requires more than 4 portals to solve.