My kids have a monthly cycle, one where they get along amazingly well for about three weeks, then for a week or so they act as if they are rival enemies and want to strangle each other at the slightest errant glance. Basically when they are good they are very VERY good to each other, and when they are enemies they are horrendous.
Tonight they are, without a doubt, awesome. They chose imaginary play over video games and movies, and ever since they got home from school they have been battling something-or-other, with a rubber knife and empty squirt guns. I do my best to stay out of their playtime and do my own pretending - I pretend I'm not paying attention. If they see me watching, their bubbles burst ever so slightly and their imaginary worlds collide with reality and their fun dampens just enough to not be so fun anymore.
So as I make dinner (and finish typing this post), I will not be paying attention to them shooting their guns (pchew pchew PCHEWWW), "covering" each other, walking up the stairs backwards with their weapons drawn, hiding in their safe hideouts from *them,* declaring the coast clear. I will not be paying attention to their giggles, or to how they help each other, or how they are planning to hide from the intruders. I will not, under any circumstances, let them know how happy it makes me feel that they are getting along so well because then their secret will be out. Don't want to ruin a thing.