If you're referring to the cap on a three-piece, leave it on ... it'll happily tap away. Just fill it so the liquid is between the "fill" lines and you're good to go.

Since this is a airlock thread I thought would add my latest misshap.....I have a 5 gallon ferment going, with the three piece airlock. I stopped by to check on my jug and when I opened the door instead of hearing -glug/glug/glug/, I heard a sizzleing roar. Oh damn I thought, as I entered I found that I filled my fermenter juuuust a little too full and pushed my airlock juuuuust a little too far down into the fermenter. Yep you got it, mash was erupting from the airlock and had made a mess from the top to the bottom and down into the drain of the bathtub. It did smell kinda good in there though. So I remove airlock, wipe off lid, wipe up tub floor and reset the airlock. Shes happily glugging away now. Lesson learned, slow the hell down and pay attention. Oh god what we do for enjoyment!!!!!

A few months ago I placed my bucket lid on my wash and sealed it real nice then I went and filled up the airlock and brought it back to the fermenter I pressed the airlock onto the lid. I didn't realize it at the time but I pressed hard enough to cause the little black gasket to push through and fall into the wash. I tried fishing it out but no luck. luckily I had another lid that I wasn't using so I borrowed the gasket from it. Now I put the airlock onto the lid before I place it the lid on the bucket.