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You can be interested in something or someone, and someone or something can appear interesting to you.

Interested is the past participle of interest. When used as an adjective it shows that someone is interested in something.

For example: "The interested party contacted his estate agent."

However, verb participles can be used as adjectives. so the word "interested" often doesn't appear before the noun.

For example: "I was very interested in the lesson."

Interesting is a present participle. When used as an adjective it describes the people or things that cause the feelings.

For example: "It was an interesting lesson ."