The Manhattan Project had to find solutions to two difficulties. The first difficulty is how to make the special isotopes of uranium (uranium-235) or plutonium. This process is called separation and is very slow. The United States built very big buildings with three different kinds of machine for separation. They made enough fissionable special isotopes for a few nuclear weapons. The second difficulty was how to make a bomb that will produce a big nuclear explosion every time. A weapon with a bad design can make a much smaller nuclear explosion. This is called a "fizzle". In July 1945, the Manhattan Project solved the two difficulties and made the first nuclear explosion. This test of a nuclear weapon was called "Trinity" and was a success.