If you have two stones; one large and one small, and you push each with the equal amount of force, the small one will go faster. This means that less mass equals higher speed. No matter how tiny mass an object has, it won't have less mass than light, which is massless. Therefore, no object with mass can reach the speed of light.
http://www.youtube.com/watch?v=4B_x1XjBhiI