where does copyright law draw the line? i ask because i have heard that specifically, it is illegal to recite happy birthday for commercial purposes. so, to us

where does copyright law draw the line? i ask because i have heard that specifically, it is illegal to recite happy birthday for commercial purposes. so, to use it on your YouTube video, for instance. but as far as i'm aware, Lady Gaga is well under copyright but i'm not breaking any laws by singing along when it's on the radio. so i don't see why it would be unlawful to sing happy birthday in a private setting, copyright or not. is my logic or information flawed?