One natural question to ask is whether a given finite tree (that's a tree with finitely many nodes) contains a smaller given tree, as illustrated in figure 3. Taking this a step further, if you've got a whole collection of finite trees, what are your chances that one of the bigger ones contains one of the smaller ones? Surely this chance should increase as your collection of trees gets larger? In the 1960s the American mathematician Joseph Kruskal proved a result which confirms this intuition: as long as you've got infinitely many finite trees in your collection, you can be sure that one of them contains another.