Functional modelling is a key part of several well known engineering design methods. All stress the importance to address the form of a product separately from what the product does or could do. However, if one asks designers in companies about functional models, they rarely can provide examples, in spite of often having tried to build models. An experiment, in which 20 graduate designers were asked to analyse an hydraulic pump, has revealed that they had very different notations of functions and produced rather different functional models of the same product. This would indicate that designers struggle with functional concepts in practise and use different notations opportunistically.