As a result, development of Fat Man was given high priority. Chemical explosives were used to implode a sub-critical sphere of plutonium, thus increasing its density and making it into a critical mass. The difficulties with implosion centered on the problem of making the chemical explosives deliver a perfectly uniform shock wave upon the plutonium sphere— if it were even slightly asymmetric, the weapon would fizzle. This problem was solved by the use of explosive lenses which would focus the blast waves inside the imploding sphere, akin to the way in which an optical lens focuses light rays.