What is an example of a false statement you told someone naive that they believed?

There was a time when I told a particularly gullible friend that if you whispered to plants, they would grow faster due to the vibrations of the sound waves stimulating the plant to be more productive. I explained it convincingly, saying that some scientific studies backed the claim, which encouraged him to spend extra time having conversations with his potted plants. Though there’s truth to plants responding to sound due to vibrations potentially affecting growth, the idea that whispering would directly accelerate their growth was exaggerated for fun. My friend accepted it at face value, often recounting his conversations with his ‘responsive’ plants until I finally confessed that it was more of a parallel of sound-based studies on plant behavior rather than an established fact about whispering specifically.