Let the length of the tree be l.

As shown in the figure, put the stick BB' (reference) and the small plane mirror O on the same line with the tree AA', and let a classmate see that the vertices of the tree and the stick are A' and B' respectively through the plane mirror at the other end. Then measure the length of OA, OB and BB' with a tape measure. According to the triangle similarity principle AA'/BB'=OA/OB, so the number AA' l=OA×BB'÷OB's height.