Marcell Ateman can sense the difference.
Playing college football is hard work, but there's the less serious side of college life, being surrounded by friends you've come to know over a few years.
Now, football is business.
The Oklahoma State receiver has spent the last several weeks training at Michael Johnson Performance in McKinney, Texas, as he readies himself for the NFL scouting combine this week in Indianapolis.
The physical requirements are much the same as football season, but there's money on the line now. A few hundredths of a second in his 40-yard dash time, or some other speed drill, could be worth hundreds of thousands of dollars.