A brain expert explains the cognitive test used to assess a president's mental fitness. It's not easy.

Evan Vucci/AP Images, temet/Getty, Tyler Le/BI

• A psychologist who checks business leaders and doctors for cognitive decline shared how he does it.
• His assessment takes about an hour, and involves a lot more than just one test. 
• It's pretty challenging. 

Most Americans think 81-year-old President Biden is just too old to be president.

There are also concerns about 78-year-old Republican nominee Donald Trump's fitness for office — registered voters are...