A great idea while sober is associated with a light bulb. A great idea while drinking is known as a beer bulb. It's as if a glass of beer lights up over your head.
Alex: I just had a great idea. What if we sing Celine Dion to win Karaoke to pay for our bar tab tonight?
Marilyn: Brilliant! Your beer bulb just went off! My heart will go onnnn!