This is What's Trending Today.John Urschel was living a life many athletes only dreamed of.He was a player in the National Football League (NFL).Until Thursday, the 26-year-old Urschelplayed for the Baltimore Ravens football team.But not anymore.He announced he was giving up football to study mathat the Massachusetts Institute of Technology.Urschel played in the NFL for three years.He came to the Ravens after playing in collegeat Penn State University in State College, Pennsylvania.Urschel was known among American sports fansfor being both a good football player and also a mathematician.Urschel studies complex math,like advanced algebra and machine learning.He wrote about his studies in a 2015 article in The Player's Tribune.It was called "Why I Still Play Football."He wrote that he loves math, but also loved to play football."There's a rush you get when you go out on the field,lay everything on the lineand physically dominate the player across from you."Urschel was an offensive lineman.That means he used his large body– over 1.9 meters tall and 136 kilograms– to keep the opposing team's players from tackling his teammates.But it also means that he ran into other players over and over again.Earlier this week, a study was releasedabout the brains of American-style football players.Researchers examined the brains of 111 former NFL players who had died.All but one of them showed signs of chronic traumatic encephalopathy.That is the brain disease known as CTE.It is linked to hits to the head that are common in football.The brains were donated by families of the former football players.The Baltimore football team posted the news on Twitter.The team just started practicing for the new season this week.Baltimore's coach is John Harbaugh.He said: "we wish him all the best in his future endeavors."Urschel has not yet talked about his retirement.But the announcement came after the new study was released on TuesdayUrschel is not the only player to announce his retirement this week.Andrew Hawkins was a wide receiver for the New England Patriots.He retired on Tuesday at the age of 31,announcing his plans to study business and economics.Hawkins earned a master's degree from Columbia University earlier this year.Many people reacted to Urschel's news on Twitter,calling him a "genius" and praising his decision.And that's What's Trending Today.