I will discuss the dynamics of Nambu--Goto strings with junctions at which three strings meet, in particular, the process of intercommuting of two straight strings, in which they exchange partners but become joined by a third string. I shall show that there are important kinematical constraints on this process. The exchange cannot occur if the strings meet with very large relative velocity. In the case of non-abelian strings, there is also a possibility that they can get locked, unable to separate in any direction. This may have important implications for the evolution of cosmic superstring networks and non-abelian string networks.