The 'What Do You Hear?' TikTok challenge centers around auditory illusions where a single audio clip sounds like two entirely different words or phrases depending on which one the viewer focuses on. One of the most famous examples includes the 'Brainstorm' or 'Green Needle' toy sound, which gained massive popularity in 2020. This phenomenon, known as top-down processing, demonstrates how the brain uses visual cues to interpret ambiguous sounds. Users typically post videos with text boxes for both options, asking their audience to comment on what they hear, sparking endless debates and high engagement across social media platforms like TikTok and Instagram.