A proton strikes an antiproton head on. They both smash to bits, and pieces fly off in every direction. That doesn't make sense. Because of the connection Einstein found between matter and energy.
E stands for energy and m stands for mass.
The two quantities are tied to each other. Accelerating the proton and the antiproton to almost the speed of light gives them an enormous amount of energy. When they collide, most of that energy can turn into a corresponding amount of mass in the form of the particles that fly out of the collision. What a guy.