A Weather balloon filled with helium will fly for the same reason a cork will rise to the surface of water if you let it go under water. The cork is lighter, or has less density, than the fluid or water that surrounds it. The same is true for a balloon filled with helium. Helium has less density than the air in our atmosphere, causing it to want to float to the surface of our atmosphere. This is a good thing for us because the surface of our atmosphere is where space begins and where we want to fly. By providing the weather balloon with enough buoyancy (ie. enough helium) we can attach a line to the balloon, called the flight train, with our payload suspended at the other end. The balloon will then lift our payload (YOUR LOGO) to the edge of space.