Why can a splash of pasta water make oily or cheesy sauce cling to noodles?

Answer: Starch binds the sauce

Water dilutes the fat — No. Plain water can loosen a sauce, but dilution alone often makes it thinner and less clingy. The reason pasta water works better is that it is carrying starch, so it can both adjust thickness and stabilize the sauce. That is why clear tap water is not an equal substitute.

Salt melts the cheese — Not really. Salt improves seasoning, but the cited pasta-water tests point to starch, not salt, as the binding helper. This choice confuses flavor with sauce structure. The useful trick is cloudy water carrying starch into the pan.

Starch binds the sauce ✓ — Right. Pasta water is not just hot water; it carries starch released from the noodles. Cooking guides and tests describe that starch as helping sauce thicken, bind, and cling. The surprise is that a cloudy waste-looking liquid can act like a tiny bridge between sauce and pasta surface.