Give an example of commonplace assertion

Milk is good for you.

America is the most free country on Earth.

You will get cramps if you swim right after you eat.

those are examples of Coomonplace assertion

A commonplace assertion is a statement that people just assume is correct and applies to everyone, even though there may not be any proof to support that assertion.

For example: An apple a day keeps the doctor away.

This asserts that if you eat an apple every day you won't ever get sick, and that is certainly not true.