“There was a theory among linguists that the brain’s capacity for language learning—language as a concept, a modality for thought—is finite. Scientists called the period from ages zero to five the “critical window,” within which a child had to gain fluency in at least one language, any language, or risk permanent cognitive damage. Once the window shut, learning anything became difficult, even impossible—without a language, how does one think, or even feel? The critical window remained “theoretical,” mostly because intentionally depriving children of language was deemed by ethicists too cruel an experiment to conduct. And yet, February saw the results of such trials every day—children whose parents had feared sign language would mark them, but who ended up marked by its absence. These children had never seen language as it really was, outside the speech therapist’s office, alive and rollicking, had never been privy to the chatter of the playground or around the dinner table.”