Adventure Comics #189

Professor Lang returns from an island like Easter Island with a belt that incorporates alien technology. Lana Lang dons the anti-magnetic and anti-gravity belt to become Sky Girl. Superboy uses Lana's new super-powered identity as a chance to pay Lana back for her secret-identity snooping. In the end Superboy gives Lana a super-compressed pebble with HIS secret identity written in small letters on it. When Lana sets down the pebble the resulting crash alerts Superboy to Lana's dual identity. After all, she does live next door.