(CNN) If you know one thing about last week's presidential debate, it's probably this: Julián Castro seemed to question Joe Biden's age and mental acuity by repeatedly asking the former vice president if he was "forgetting" something he had said two minutes prior.

And that was important! But it's what happened after the debate that actually might tell us more about where the race is headed -- and how soon.

In an interview after the debate, New Jersey Sen. Cory Booker was asked by CNN's Erin Burnett about Castro's hit on Biden. And Booker responded this way:

"I think that we are at a tough point right now because there are a lot of people who are concerned about Joe Biden's ability to carry the ball all the way across the end line without fumbling. And I think that Castro has some legitimate concerns about, can he be someone in a long, grueling campaign that can get the ball over the line. And he has every right to call that out."

Which is, well, verrrrrrrry interesting. Here's why.

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