Why do the kids spit on the gate when they sneak into Boo Radley's yard?

In chapter 6, the children raid the Radley yard at night in an attempt to peek through the windows and get a good look at Boo Radley. When the children approach the Radley yard from the rear, they must first crawl underneath the wire fence surrounding a large garden and outhouse. After crawling underneath the wire fence, the children quietly walk through the garden and Scout is cautious not to step on any of the loud collards. When they come to the gate that separates the garden from the back yard, Jem goes to open it and discovers that it squeaks. Dill then suggests that everyone begin spitting on the hinge of the gate to make sure that it will open without making a loud noise. After spitting themselves dry, Jem is able to quietly open the gate and the children furtively approach the Radley house.