In this lecture we gained some first experience with scientific programming in Python. We used a simple example, the radioactive decay of a number of atoms to introduce some elementary techniques for the solution of ordinary differential equations of first order, the Euler method. We discussed the algorithm for the numerical solution and the structure of the corresponding code with some pseudo-code, and we also discussed the emerging code. By running this code we had a first encounter with one of the sources of errors in such numerical/computational solutions, namely the discretisation error.