Kiki is so right. They tell you in the books, etc. that if the lid comes off, you are done. Your babies will all die... DIE!!!
This is like telling your child, "If you go down to the creek without your older sister, Jenny Green-teeth will come out of the water, drag you in and EAT you!!" It is a cautionary tale. It's best not to open it once the eggs begin to pip, but if you gotta, then you gotta.
Oh BTW, in case you don't have an easy way to add water already (the sponges are another matter of course), I found a syringe and tubing as a set on Amazon and I just add through the vent. I can direct the end to the channel I want by just twisting it around. Don't forget to disinfect after your chicks have hatched. I just draw up some of the original gold Lysterine. I use it as a spray on the (plastic) incubator as well, and diluted as an egg spray if I feel the need for some reason to clean the eggs.