Subjects at four age-levels (7 years, 11 years, 14 years, adults) were asked to complete sentences of the type 'my heart is like . . .'. Seven examples were given (heart, lungs, brain, body, germs, blood and stomach). There was a decline with age in the number of subjects using shape and colour as the basis for these analogies, and an increase with age in subjects using analogies that describe functions. The data are discussed in terms of their implications for explaining illness to sick children. It is argued that explanations involving functional analogies may be less readily understood than previously supposed.