The riddle
What Has Four Eyes but Cannot See?
Count the i
Words that sound alike but are spelled differently are called homophones, and the word eye and the letter I are a pair. Write Mississippi out and there are eleven letters: one m, four i, four s and two p. The four i's are the four eyes, and the word cannot see. People also type the question as "what has 4 eyes but can't see", and the word that fits does not change. A few other words reach four, such as civilization and artificiality, but Mississippi is the famous one.
The river behind the word
Wikipedia gives the length of the Mississippi River as 2,340 miles (3,766 kilometers), a U.S. Geological Survey figure. It traditionally starts at Lake Itasca in Minnesota and ends in the Gulf of Mexico in Louisiana. The river runs through or borders ten states. The name comes from the Ojibwe misi-ziibi, meaning great river.
The state that took the name
Mississippi became the 20th state on December 10, 1817, and Jackson is its capital and largest city. Both the river and the state carry the same eleven letters, so the riddle works for either one. Nothing in the question says which is meant, and what fits is the word, not the place.
Another counting riddle
Letter counting also drives what has four letters, sometimes nine, but never five. The two ask the listener to look at the words themselves and not at the thing they name. Try telling this one by having the group spell the word aloud while a helper tallies the i's on a board.