He was the (clever)＿＿＿thief of all.

__Question__He was the (clever)＿＿＿thief of all.

The answer is

**cleverest**but i don't understand why the answer is

**cleverest**and not than

**cleverer**.

__I know the question is sort to superlative.__**BUT I JUST DON'T UNDERSTAND!!!**

**(Please someone help me.)**

**thank you**