Former President Trump took aim at President Biden after he appeared to stumble over his words while answering a question during Thursday’s debate.

“I really don’t know what he said at the end of that sentence. I don’t think he knows what he said either,” Trump said when he began his response to Biden’s comments.

CNN’s Jake Tapper asked Biden why should voters “trust” him to resolve the crisis at the border, noting that a record number of migrants have crossed into the U.S. during his administration. Biden pointed to the bipartisan border security deal that Republicans blocked and his recent executive order that limits the number of migrants coming across the border once the number hits a certain threshold.

He then appeared to misspeak before concluding his response.

“I’m going to continue to move until we get to total ban — on the total initiative relative to what we’re going to do with more border patrol and more asylum officers,” Biden said before Trump took aim at him.

Earlier in the debate, Biden appeared to lose his train of thought on an answer about Medicare.

Biden and Trump are going head-to-head in the first presidential debate of the 2024 election cycle Thursday night. The stakes are high for both candidates as they make their appeal to voters in what will likely be another close election.

Biden, 81, and Trump, 78, are the two oldest major party candidates to run against each other. While Trump is only a few years younger, polls have shown that voters are more concerned about Biden’s age and mental fitness for office.