Take a clip-on light (or a similar light) and select an object to draw. Illuminate the subject being drawn, then slowly move the light to different positions, observing how the patter of shadow and light shifts as the direction of the light source changes. Draw your object 3 times (on 3 separate pages), and make sure the lighting is noticeably different each time. Use graphite and shading (rather than stippling or hatching).
Make sure that you fill the entire page with each drawing (avoid small in the middle of the page). Show evidence of time invested – it is very easy to tell when a student has spent 10 minutes on a drawing versus 30 minutes.
Image Credit: Phil Jacob