On hearing on Go, Diego, Go! that Pygmy marmosets are the smallest monkeys in the rainforest, The Five-Year-Old asked: “What are the tiniest monkeys in the world?”
According to FactZoo, the Pygmy marmosets can claim that title too. (Although pygmy mouse lemurs and pygmy tarsiers are slightly smaller, they are considered primates, not true monkeys.)
Answering this question opened the door to a whole new world of cute on the Intertubes. The Five-Year-Old’s favorite find so far? The collection of the World’s Smallest and Cutest Animals at ItThing. But I expect that’s just because it includes a 3-pound cat.