Eleonore Pauwels is right that biologists should not mislead themselves or the public by using simplistic metaphors borrowed from engineering (Nature 500, 523–524; 2013). But biologists don't simply borrow words, they take engineering principles — derived from theory and practice — and apply them to biological systems.

Referring to the heart as a pump or to the nasal passages as heat exchangers is not a simple metaphor like calling Juliet the Sun. We use the same terms because the same formal criteria can be usefully applied to both engineered and evolved systems.

Shared engineering terminology extends beyond biomechanics to molecular and systems biology. For example, genes have been successfully modelled as Boolean logic switches to predict gene expression in the developing embryo (I. S. Peter et al. Proc. Natl Acad. Sci. USA 109, 16434–16442; 2012).

In my view, it is an oversimplification to think that all engineering talk in biology is mere imagery.