A small grey box filled with a various bits of tree. For keen plant scis mainly, but we think you could use it to explain transport of water/nutrients in the xylem and sugars in the phloem. You can also talk about aging a tree by counting the number of rings present, and how you can tell when the tree lived by comparing the relative size of each ring to trees of known age (thicker rings correspond to favourable growing conditions, while thinner rings correspond to a harsher climate).