In an enzyme reaction the rate rises while the varied factor limits it, then plateaus once something else becomes limiting. Read the plateau, not just the slope: reaching a flat maximum tells you the varied factor is no longer the limiting one, and for a product-against-time curve the height of the plateau is set by the total substrate supplied.
Trigger: a curve that levels off, or a question naming a limiting factor.
Instances:
(i) rate against substrate concentration flattens when every active site is occupied (saturation): the rate is at its maximum, so more substrate changes nothing and doubling the substrate does not always double the rate;
(ii) rate against enzyme concentration flattens when the enzyme is no longer limiting: the substrate now limits, so adding substrate would raise the rate;
(iii) a product-against-time curve levelling off means the substrate has been used up (or the product now inhibits the enzyme), never that the enzyme was used up;
(iv) two product-against-time curves reaching the same final height were given the same total substrate, whatever their speeds.
