Wikipedia defines common sense as “knowledge, judgement, and taste which is more or less universal and which is held more or less without reflection or argument”
Try to avoid using this topic to express niche or unpopular opinions (they’re a dime a dozen) but instead consider provable intuitive facts.
Pretty much anything related to statistics and probability. People have gut feelings because our minds are really good at finding patterns, but we’re also really good at making up patterns that don’t exist.
The one people probably have most experience with is the gambler’s fallacy. After losing more than expected, people think they’ll now be more likely to win.
I also like the Monty Hall problem and the birthday problem.
The immune system is strong and defends your body against germs.
The immune system works 100% of 50% of the time. Immunology is the best way to convince someone that it’s a miracle that they’re still alive. Anyways, get vaccinated. Don’t rely on your immune system to figure things out
Another variation of that is claiming how getting sick repeatedly is somehow beneficial for getting a strong immune system. That ignores research, as children who have a lot of common infections early in life have higher risk of moderate to severe infections and antibiotic use throughout childhood. That also ignores viruses for which a durable immunity isn’t currently possible, such as COVID.
EDIT: Basically the immunity system doesn’t work like a muscle.