"Only know how to say ..."

"Don't know how to say dick," Isaac said this morning.

He's been trying to say this word for a while now, but he's stymied by his inability to pronounce the letter S. What he wants to say is "disc," as in "disc harrow," the attachment a tractor pulls behind it to cultivate the soil.

"Only know how to say plow," he said sadly.

Then he brightened, and added, "And tractor. And truck. And van. And cherry-picker. And car."

This list of the vehicles he does "know how to say" went on for quite a while. When he paused, I pointed out to him that he does know how to say disc, but that his tongue won't cooperate with his knowledge. (I did not, however, point out that his pronunciation of the other vehicles is similarly compromised: truck is tluck, van is han, and car is cow.)