If humans are ever going to have a long-term presence on Mars, they'll need to make their own buildings -- they can't count on timely shipments from Earth. But how do they do that when the resources they have will share little in common with what they knew back home? Northwestern University researchers have an idea. They've developed a concrete that uses Mars' native materials. You only have to heat sulphur until it melts, mix it with an equal part of Martian soil and let it cool. The finished concrete is very strong, easy to work with and recyclable -- you just have to reheat it to get some building supplies back.