The term 'WHAT PUPILS DO IN THE DARK' can be solved by the word 'DILATE', which is a six-letter word. Dilation is a process in which an object or area is made larger or wider. In the context of pupils, dilation refers to the widening of the pupil of the eye when exposed to darkness. This is an involuntary response of the eye to adjust to the lack of light, allowing more light to enter the eye and improve vision in low-light conditions.