Why Does Milk Seal Sidewalk Chalk?

I work at a public library and we recently had a sidewalk chalk hour for families. Someone gave us a tip that if we brush milk with a paintbrush over the finished chalk drawings, it helps seal them and makes them last longer. It has been several weeks, two heavy rainstorms, and a hail storm, and all of the drawings we brushed with milk are still there! The ones we didn't brush with milk are long gone. What are the chemical properties of milk that make it do this to sidewalk chalk?