A fissile material is one that can support a fission chain reaction. Uranium-235 and plutonium-239 are the fissile materials most often used in nuclear bombs, and producing or procuring them is usually the most difficult part of a weapons development program. During the Manhattan Project, for example, around 90% of the total program budget was devoted to the production of U-235 and Pu-239. Modern pits (see below) often combine the two.