What is the slowest mammal on earth?

I'm sure its a three-toed sloth. They only come down from a tree once a week. Scientists who study sloths are always falling asleep - I'm not joking! Imagine watching a furry green thing do nothing for hours on end? The first thing a scientist who first saw the sloth said was: "I have never seen an uglier or more useless creature." But still their faces are quite cute I have to admit.