Two pale green plates of glass sand form a channel. Drag the upper or the lower plate, or pinch to change the gap, and both height and width move. A grain through the opening is caught. A horizontal blade is safe only when the whole blade fits the gap. A graze costs a life, and you have three. Glass can stick to the finger, and some grains flash and vanish. Cool bright grit reflects. Letting a blade through and catching a grain often share one breath. Later channels only make that gap longer and stickier.
Slide twin glass plates, catch sand grains, and let blades pass.