// the rand() function creates a reproducible sequence 
// of pseudo-random numbers between 0.0 and 1.0

#declare R1 = seed(0);      // initialize random number streams
#declare R2 = seed(12345);

// place sphere with random radius at random position
// use different random number streams for position and radius
sphere { <rand(R1), rand(R1), rand(R1)>, rand(R2) }