A cell often must answer a smoothly graded chemical signal with a clean yes or no. The MAPK cascade — Raf switches on MEK, MEK switches on ERK — is one machine that manufactures that decision.
Each layer is a tug-of-war: a kinase adds a phosphate to activate the next protein, a phosphatase strips it off. When both enzymes run near saturation, flat-out, the layer's output snaps from off to on over a razor-thin change in input — the zero-order ultrasensitivity of Goldbeter and Koshland.
Stack three such layers and the steepnesses multiply. James Ferrell measured an effective Hill coefficient near 5 in frog eggs — steeper than any single molecule can be. The switch is emergent: no enzyme here is itself a switch.
Turn the phosphatases down and the very same wiring becomes a gentle dimmer that merely tracks the signal. Steepness is something a cell tunes, not something built in.
The simulation stopped unexpectedly — the lesson continues without it. You can move on; nothing you did was wrong.