Worldchanging.com founder Alex Steffen offers a fast-paced round-up of radical (but possible) answers to our planet’s greatest challenges, ranging from green cities and buildings, to digital collaboration tools, to ingenious tools for the developing world (flowers that detect landmines; straws that purify water as you drink; merry-go-rounds that pump water using the energy expended by children at play). As Western-style consumerism spreads to developing countries, we must re-imagine our world.