Not everything has to be explicitly taught in school or via formal lessons. Observation, trial and error are sometimes all it takes for the motivated "Ape" student.
Koko blows over a blade of grass that she holds stretched between her two thumbs, to make a whistling sound. Sometimes she produces a nice high-pitched whistling sound, and sometimes she doesn’t. Either way, Koko has fun trying. She even signs “good” to indicate she approves of her efforts. How did Koko learn to do a grass whistle? By watching Penny, of course — the same way kids have learned new things for years, in fact thousands of years, even before there were schools.