Your burner can't make boiling water hotter. A lid can.

Quick answer: A sealed pot traps steam, the pressure climbs, and higher pressure raises the temperature at which water boils. At 15 psi that's about 250°F instead of 212°F. Hotter water cooks food faster. The burner just keeps the process going.

Turn a burner all the way up under a pot of boiling water. The water will thrash, steam will pour off, and the temperature won't rise a single degree. At sea level it sits at 212°F, and it stays there.

Your potatoes cook at the same speed in a lazy simmer as in a furious boil. The extra flame only boils the water away faster.

Why boiling has a ceiling

Boiling is a thermostat nobody told you about. Extra heat from the burner doesn't make the liquid hotter. It turns liquid into steam, and the steam leaves, carrying that energy with it.

So the water is pinned at 212°F. The only ways to cook faster in an open pot are to cut the food smaller or to be patient.

The way around a ceiling is to move it.

What the lid changes

Water boils when its vapor pushes back as hard as the air pressing down on it. At sea level, that air pressure is about 14.7 psi, and water gets there at 212°F.

Seal the pot and the air pressure is no longer the only thing pressing down. Steam can't escape, so every bit that forms adds to the pressure inside. Now the water has to get hotter before it can boil, which makes more steam, which raises the pressure further. It's a loop, and it stops when the valve says so.

A typical stovetop cooker holds about 15 psi above the surrounding air. That's roughly double normal pressure, and it puts the boiling point near 250°F. Most electric multicookers run lower, around 10 to 12 psi, which lands closer to 240°F.

The valve on top, a weight or a spring, lifts to release steam whenever pressure goes past its setting. That's the hiss or the jiggle. Once it starts, turn the burner down to the lowest setting that keeps it going. More flame doesn't raise the temperature. It just wastes steam and water.

The idea isn't new. A French physicist named Denis Papin built what he called a steam digester in 1679, mostly to soften bones into something edible. He's also credited with adding the safety valve, which turns out to be the part that matters most.

Thirty-eight degrees, four times as fast

Going from 212°F to 250°F doesn't sound dramatic. It is.

A rough rule of thumb in chemistry says reaction rates about double for every 18°F rise. Thirty-eight degrees is a little over two doublings, so about four times as fast. Real food isn't one tidy reaction, but the kitchen results line up with it.

A pot roast that wants three hours in the oven is done in under one. Unsoaked dried chickpeas that take two hours on the stove finish in under an hour. What takes the time in those foods is collagen, the tough connective tissue that has to melt into gelatin. Heat it harder and it melts faster.

Be honest about the clock, though. The cooker needs ten to fifteen minutes to build pressure before the timer even starts, and more time to release it afterward. You save the most on foods that take a long time anyway. A pot of rice or a few carrots barely come out ahead.

Altitude is the other half of the story

In Denver, at around 5,280 feet, water boils near 202°F. Beans take forever and pasta needs extra minutes, because the thermostat's ceiling is lower.

A pressure cooker still lifts the temperature well past what an open pot can reach there. It just starts from a lower baseline, so the result is a bit less than at sea level. That's why most manuals tell you to add some time if you live well above a couple thousand feet.

What goes wrong

Pressure cooking has real downsides, and they're the same every time.

• No browning. A wet, sealed pot doesn't brown meat. Sear it first with the lid off, then add the liquid and seal up.
• Thin sauces. Nothing evaporates, so liquid doesn't reduce. Once the food is out, simmer the sauce uncovered for a few minutes.
• No liquid, no pressure. Without water there's no steam, and without steam there's no pressure. Your manual sets a minimum, usually around a cup. Respect it.
• Foamy foods. Beans, oatmeal and pasta foam up and can clog the vent. Fill the pot no more than halfway with those, and no more than two-thirds with anything else.
• Fast overcooking. One extra minute turns broccoli into a sad green paste. Use short times and a quick release for vegetables.

Why it's safe, and how to keep it that way

An airliner is a much bigger pressure vessel, holding in a cabin at a differential of around 8 psi, as the physics of flight quietly depends on. A pressure cooker holds nearly twice that in a pot you can lift with one hand.

That works because it's built the way a bridge is built: with large margins. A modern cooker has a gasket, a primary valve, backup vents and a lid that locks until the pressure is gone.

Your part is simple. Never force a lid. If it won't turn, there's still pressure inside. Wait for the float pin to drop or the hissing to stop. When you do release steam, aim the vent away from your face and hands, because that steam is around 250°F and it burns instantly.

Keep the gasket clean and replace it when it's cracked or stiff. A gasket is a ten-dollar part. Treat it that way before it becomes a scalded forearm.

One more thing the ceiling explains

That 212°F ceiling is also why boiling jars of green beans doesn't make them safe. Botulism spores survive boiling water. To destroy them in low-acid foods like vegetables and meat, home canners need 240°F or higher, and only a pressure canner gets there.

That's a job for a dedicated canner and tested guidance from the USDA. Most electric multicooker makers say plainly that their machines aren't meant for canning, and they're right. Follow the tested times exactly.

Papin was trying to soften a bone in 1679. You're doing it to a chuck roast on a Tuesday. The trick is the same: stop begging the flame for more, and change the rules the water plays by.