Nine pictures in color, each with the first and last letter already given and the middle one missing. Cat, hen, pig, mop, bus, bat, bed, six and dog, so all five short vowels turn up and none of them more than twice.
The middle sound is the hardest of the three for most children, because the first and last are easy to hear and the vowel sits buried between them. Saying the word slowly out loud is the whole technique, and it is worth doing together on the first row. If a child writes an a in hen, they are hearing the sound but not yet matching it to a letter, which is a different problem from not hearing it at all.