Because this...
took me about three hours to do.
And that was after I figured out that using any kind of roller wasn't going to work.
First, I tried to use a foam roller, which some of the tutorials suggested. It worked fine on the piece of cardboard I used for practice, but didn't cover well on the floor at all. Then I figured since that part of the floor was already ruined, I'd try to use a paint roller with a very fine nap. That covered well, but the paint seeped under the edges of the stencil and looked awful. So I wiped the teal paint off the floor as best I could with a wet paper towel and then re-painted the floor with two coats of the white.
What does work? A stencil brush. It works great. It takes a thousand times longer than a roller would, but it works great.
A couple of people asked about the durability of the floor. After I get all this stenciling done, the floor will get two coats of Varathane Crystal Clear to protect it. Remember that cool paisley floor I loved? Carrie from Lovely etc. did that--in her living room and dining room, no less!--and was kind enough to blog about it again seven months later to report that her floors still look beautiful. That's reassuring, since I'll be spending 30 to 40 hours tapping a little tiny stencil brush on my bathroom floor...