All clocks are an analog of the earth spinning on its axis, which we divide up into minutes and seconds to tell the time. But the earth's rotation is not constant, and changes speed over time. The International Earth Rotation and Reference Systems Service decides when we need to add an extra second here and there to keep our clocks in sync with the actual length of a day. Chris explains more. While many of us have heard stories of virgin births, in reality they are far more common than we might think. Claire Farrugia explains how in nature, parthenogenesis can help keep some species from falling into extinction when numbers drop below a critical level.