:Fis phenomenon:

A feature of the acquisition of speech by children. The name comes from an incident reported in Berko & Brown (1960). A child called his toy fish fɪs. When asked: “Is this your fɪs?”, he said no. But when asked: “Is this your fɪʃ“, he said: “Yes, my fɪs.” This is evidence that children’s perceptual abilities are often in advance of their productive abilities.

Berko J. & Brown R. (1960) Psycholinguistic Research Methods. In Mussen, P. (ed.) Handbook of Research Methods in Child Development , New York: John Wiley, 517-577.