The paper answers question "What is computation?" presenting it as a special case of a more general natural process involving dynamics of interacting information systems. For this purpose, a more detailed description of the generalization of Turing machine called symmetric machine outlined in author's earlier paper is used as a model for natural computing in which pro-active character of the machine with differentiated functions of its components (head and tape) is replaced by symmetric interaction of functionally equivalent information systems of hierarchic structure. Turing's A-machine is a special case of such S-machine in which only local states of cell's are changing, but not instructions on the list. The importance of the interactive character of computation is analysed in the context of exclusively interactive processes of mechanics. Causality involved in physical implementations of Turing machine can be preserved in its interactive generalization at the higher, collective level of its structure.