Females of the santa species (commonly referred to as "mrs claus" but the technical term is "Santa Hen") lay eggs in fir trees, to be fertilized by male santas ("nicks"), so you see by putting a tree in your own home decorated with shiny, fake, santa eggs, you can lure one into your home during their brief mating season.
If a Santa is lured into your home and finds it resource-abundant (easy access to milk and cookies) and safe (doesn't smell naughtiness) then it will attempt to fertilize the eggs on the tree and leave "presents" to nourish their (sadly non-existent) offspring when they hatch, which can then be trivially harvested and opened. Of course modern santas have been bred to produce more and more elaborate and valuable toys than past breeds.
