What's the relative clause in this sentence

His questions revealed an intelligence that was beyond his young years

"His questions revealed an intelligence" seems to be the main idea so what is the rest ?

http://www.chompchomp.com/terms/relativeclause.htm

The relative clause in this sentence is "that was beyond his young years." To identify a relative clause, you can look for a subordinate clause that starts with a relative pronoun (such as "that," "which," or "who") or a relative adverb (such as "where" or "when"). In this case, the relative pronoun "that" introduces the relative clause. The relative clause adds more information about "intelligence," providing a description of its quality or characteristic.