What marine mammal is known for holding hands to stay together while sleeping?

Sea otter

Which actor portrayed the Joker in "The Dark Knight"? Which dairy-based beverage is fermented with probiotic bacteria and has origins in the Caucasus region? Who led the Protestant Reformation in Scotland? Who seized power in Egypt through a military coup in 1952, becoming president from 1954 onwards? What is a sphere divided exactly in half called?