What do trees know that we don't? 13-year-old inventor Aidan Dwyer realized that trees use a mathematical formula to gather sunlight in crowded forests. Then he wondered why we don't collect solar energy in the same way.
In pictures: Mexico's butterflies
Experts say deforestation threatens the spectacular annual migration of millions of monarch butterflies from Canada and the US to forests north-west of Mexico City. Words and pictures: Kate Joynes-Burgess.