Is it trying to hit a 100 mph baseball? Considering it only takes four-tenths of a second for the ball to leave the pitcher's hand and travel the little more than 63 feet to the plate, it's probably up there.
How about playing goalie in the NHL, where slap shots can reach 119 mph (Bobby Hull) and goalies use their bodies as targets while standing on half-inch blades?
Lions head coach Jim Schwartz would like to offer up another suggestion.
"It's tough to play corner (in the NFL)," Schwartz said this week after an OTA practice. "I think it's probably the most difficult thing to do in all of sports.
"You have to run with world-class receivers, you've got to start backwards and they're going forward. They know where they're running, you don't. You've got to be tough enough to take on pulling guards and running backs and skilled enough and fast enough to cover the elite athletes, you know, guys that are Olympic-caliber speed."
Three traits make playing cornerback in the NFL a little easier: being big, fast and strong.
"We've always sort of had that in our profile, but the opportunity to get those guys (is slim) ... there are not many of them out there," Schwartz said. "There are probably a lot more 5-9 and 5-10 guys out there than there are 6-0, 6-1 guys. But all things equal, when you do have more size, it's beneficial for you."
"You don't want to end up sacrificing speed, you don't want to end up sacrificing toughness and change of direction. I think that's the balance that you have to play there," Schwartz said.
"But if you have the change of direction and have the speed, then size is definitely beneficial. I think that's been more of our emphasis, trying to find guys that have all of it. They are rare to find, something you have to develop, which we're working on with Chris Greenwood. You know, small school guy but has height and has weight and has a lot of things that we're looking for. Other guys, like (Ron) Bartell is a veteran player, we had the opportunity to get and we were able to plug them in."
Bigger and stronger hopefully means more durable, too. The Lions started 13 different combinations in their secondary last year because of injuries and ineffectiveness.
They're looking for much more consistency in 2013 and hopefully bigger, stronger and faster helps provide some.
"It's even noticeable here in practice," Lions receiver
"With size and speed on the backend of our defense, it's definitely helping."