It has to do with one of his more unusual personal habits.

"Agents say that, whether at the vice president's residence or at his home in Delaware, Biden has a habit of swimming in his pool nude," author Ronald Kessler wrote in the 2014 book "The First Family Detail: Secret Service Agents Reveal the Hidden Lives of the Presidents."

For most people, this would be a personal quirk, something they do in the privacy of their own home.

But it's quite different for someone who is under watchful eyes of the Secret Service.

There are rules, after all.

For example, presidents and vice president aren't allowed to drive.

Too dangerous.

And if they want to do something even a little risky like swim, they essentially need a lifeguard around. They need the Secret Service standing by.

But rather than put on a pair of trunks when under the gaze of his hard-working agents, Biden reportedly made them watch him swim in the nude.

"Female Secret Service agents find that offensive," Kessler wrote.

Kessler has written nearly two dozen books, including multiple best-sellers, with many offering behind-the-scenes accounts of the White House and government agencies including the FBI and CIA.

But none of them paint quite the mental picture found in the pages detailing Biden's swimming habits.