Suppose that T1 and T2 are spanning trees of a simple graph G. Moreover, suppose that e1 is an edge in T1 that is not in T2. Show that there is an edge e2 in T2 that is not in T1 such that T1 remains a spanning tree if e1 is removed from it and e2 is added to it, and T2 remains a spanning tree if e2 is removed from it and e1 is added to it.