While red squirrels store great quantities of food in large caches, grey squirrels perform what is called "scatter hoarding", burying one food item here, one there. In winter, they must then find these buried treasures and obviously they do find many. Speculation is that their sense of smell is so good that they can sniff out the nuts, acorns, etc. even when they are frozen and snow covered. In this case, the acorn was frozen into the ground and the squirrel I came across was standing in place gnawing away at the acorn. You can clearly see the teeth marks.