AMHERST, Mass. - Nearly two years ago, University of Massachusetts engineering student Nathaniel Mulcahy bumped into the bathroom sink, and his Mennen Speed Stick smashed to the floor. "What a lot of bits of plastic," the mechanical and industrial engineering senior thought as he cleaned up the mess. Then he decided to build a better deodorant stick. Mulcahy''s resulting "Speedy Speed Stick" led him to win the student contest at the 1999 International Mechanical Engineering Congress and Exposition in Nashville, Tenn., late last year.