When you walk through the garden section of your home store in the summer you will see a bunch of misters spraying the plants every now and then to keep them moist. Johnny Klaus has built a system that will mist his plants using an Arduino that is pumping a spray bottle using a 12 volt door lock motor. I think this system could also be used to keep those darn kids off my front porch, just have an Arduino count the number of door bell rings in a 10 second time frame and if it is greater than 3 give them a spray of water!