Last week, I posted some pictures on Instagram of my kiddos stacking and building Christmas trees with green plastic cups ("Solo" cups). They had so much fun trying to create the tallest tree, the widest tree, a circular tree, measuring their height next to the tree, and crashing the tree towers down. Ha. Not only is this STEM activity great for fine motor skills, it's super cheap too! I found a pack of 84 off-brand cups at the grocery store for $3.99. I also bought some name brand cups, but…