Examples:

• I've heard of her. She's an expert in international politics.
• She's an expert lip reader, and we can trust her.
• I've heard of her. She's an expert in international politics.
• She's an expert in network security.
• She's an expert in lottery-related crime.
• She's an expert in stealth.
• She's an expert at doing nothing.
• She's an expert on Thule.
• She's an expert on the desert.
• She's an expert at breaking couples.