Place this clip at the start of the setup, before the person behind is shown, then follow the reveal with a scream for shock or a Bruh for an awkward reaction. Keep it as the opening warning in the edit, and save the reaction sound for the moment the subject finally turns around.
About this sound
This short clip is a young child calmly saying 'mama, a girl behind you' in a flat, sing-song voice, with no music or added effect. The innocent delivery makes the warning feel funny and unsettling at once, as if something is about to be revealed behind an unaware person. It spread as a reusable reaction sound on TikTok, where creators place it over unrelated footage.