A Lid Changes How Fast Water Boils
A covered pot traps rising steam and heat instead of letting it escape into the kitchen, so water in a covered pot reaches a boil meaningfully faster than the same pot uncovered on the same burner. Once something is actually simmering, though, many recipes call for leaving the lid off specifically so liquid can evaporate and a sauce can reduce and concentrate — covering a simmering sauce when you want it to thicken works against you.
Sources & methodology: These tips draw on established food-science references —
Harold McGee's food-science writing, America's Test Kitchen /
Cook's Illustrated testing, Serious Eats'
technique reporting, USDA food safety guidance,
and peer-reviewed food-safety research (including UC Davis
cutting-board studies) — not reworded recipe-blog consensus.