President Biden’s mental sharpness at age 81 was one of the key questions expected to be answered during Thursday night’s presidential debate in Atlanta.

In one of the debate’s early moments, Biden stumbled toward the end of a response on why voters should trust him to solve the country’s immigration crisis:

“I’ve changed [immigration law] in a way that now you’re in a situation where they’re 40% fewer people coming across the border illegally, especially when [Trump] left office. And I’m going to continue to move until we get the total ban on the total initiative relative to what we can do more border patrol and more asylum officers,” Biden said.

In response, Trump said: “I don’t know what he said at the end of that sentence. I don’t think he knows what he said either.”

Biden’s mental acuity also came up toward the end of the debate, when Trump challenged the president to take a cognitive test.

Biden argued he’s more fit for the office than Trump.

“This guy’s three years younger and a lot less competent,” he said.