ĉio

Ĉio sits in the correlative table where the ĉi- column (all, every) crosses the -o row (thing), so it is the exact sibling of kio, tio, io and nenio. Learn the grid once and you get forty-five words for the price of nine. As an object it takes the accusative like anything else, mi vidis ĉion, which is the ending learners most often drop here. Ĉio is for things; ĉiu is for people or for picking out each member of a group.