In theories of homogeneous nucleation, it is usual to assume that a vapour contains clusters of different sizes that grow and decay by gaining or emitting single molecules. We investigate this process by performing molecular dynamics simulations of single clusters of atoms interacting via the Lennard-Jones potential. Starting from a random initial arrangement of three atoms within a distance R_c of their mutual centre of mass, we determine the probability that the cluster grows to become a decamer. The dependence of this probability on the initial trimer energy, the value of R_c and the temperature and collision rate of the surrounding vapour are investigated.