Christian McCaffrey was here. 

Now he's gone again. 

Carolina's first-round pick flew into in Charlotte this weekend to join dozens of his new teammates at head coach Ron Rivera's Bowl-A-Palooza fundraiser. But Monday morning, as the Panthers kicked off their third and final week of OTAs, McCaffrey was already back in Denver. 

"It sucks," he said, "it's really tough." 

Because there's still a good amount of confusion among fans as to why McCaffrey can't be on the field right now, here's a brief explainer:

• According to the NFL's player personnel policy, rookies are prohibited from fully participating in their team's offseason workouts until the last day of their school's final exams.
•   The well-intentioned but outdated rule is designed to prevent rookies from leaving school early.
•   It affects students who are still taking classes and those who are no longer enrolled, like McCaffrey.
•   His school, Stanford, wraps up final exams on June 14.
•   So McCaffrey will be back in Charlotte and on the field next Wednesday, which means he'll practice twice during the Panthers' three-day minicamp. In the meantime, he'll keep working out by himself during the day, and at night, watching Rockies games and going over parts of the playbook with running backs coach Jim Skipper.

"He'll quiz me over the phone, and then I'll go over everything, and he'll tell me what to learn for the next day," McCaffrey said. 

"You can still get the plays down and get good training when you're home, but nothing will ever compete with actually getting the reps on the field, being with the guys.

"It is what it is. That's the rule, and we'll just have to deal with it."