Well, Clark hadn't been expecting to have to prove it. Usually in the Nexus, Clark Kents are a dime a dozen, so people believe him right off.

He thinks for a moment, then zips off at superspeed, blowing up a huge gust of wind in his wake. He's back about a second later with an unlit candle, and once the air settles, he stares at the wick, his eyes flaring gold, and it bursts into flame. "I can juggle the furniture if you want, but the ceiling is a little low."