1) The French and Italian words for head—tête and testa, respectively—come not from the classical Latin word, caput, but (as I understand it) from Vulgar Latin testa, which originally meant a brick and later a clay pot or jug.
2) Unrelatedly, as those of you are around small children will know, a common mispronunciation is to pronounce /k/ as /t/, for example, (puddy) tat for the animal that says "meow".
Now the amusing thing for me is that 2) leads Amy to pronounce Kopf "head" as Topf "pot"! As in, "Mein Topf tut weh!"
The obvious conclusion is that the meaning shift in Vulgar Latin which led to the French and Italian words for head was influenced by baby-talk from neighbouring Germanic tribes.