How do Tankless / On demand water heaters heat so fast?

How do Tankless / On demand water heaters heat so fast?
I have never understood how they can heat enough water instantly to wash my hands, much less heat enough water to fill a bathtub instantly.
If I put a pan of water on my gas stove and turn the flame to high, it takes quite a few minutes to bring that pot of water to a boil. (And that's just one gallon or water or less)
If I turn on the power to a standard 4500W 240V 30 gal. electric water heater filled with cold water, it takes a good hour or more to heat that water to 120deg or so.
A 30 gal gas water heater, heats faster than electric but it still takes at least a half hour if the water is cold to begin.
I have an electric element called a "bucket heater". It uses 1500W at 120V. It's made to heat a 5 gallon bucket of water, in a barn or where there is no hot water at the tap. That thing takes over an hour to heat 5 gallons of water to around 100 deg.
With that said, I cant understand how a "tankless" water heater can possibly heat enough water INSTANTLY to fill a bathtub, or even a half gallon to wash my hands. Whether they run on gas or electric, how can they surpass the speed of heating water above any of the methods I mentioned above? (And do this continually)?
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On Thu, 07 Apr 2016 03:52:45 -0400, snipped-for-privacy@unlisted.moo wrote:

It is all in the surface area contacted. The amount of water in there is low compared to the heating surface and they put a lot of power behind it all.
Put a pot of water on the stove and see how long it takes. Then dump it out and put just 1/4" in the pot and see how fast it heats.
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On Thursday, April 7, 2016 at 4:52:57 AM UTC-4, snipped-for-privacy@unlisted.moo wrote:

It's called either a BFB or BFHE.
The last B is for burner, the HE is for heating element, you figure out the rest.
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On Thu, 07 Apr 2016 03:52:45 -0400, snipped-for-privacy@unlisted.moo wrote:

The actual amount of water heated in GPM is relatively small and they use a buttload of power. (at least 5 times what your tank heater uses). My brother in law wanted tankless and he ended up with 3 heaters, propane powered, One for the kitchen and one for each bathroom. You can take a shower but the flow restrictor makes the tub fill slower than you could do with a tank heater. I think 3 heaters was overkill since I am not sure his well can even pump water that fast tho.,
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+1
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On Thu, 07 Apr 2016 03:52:45 -0400, snipped-for-privacy@unlisted.moo wrote:

They require huge amoints of amps or cu ft of gas - to the point many houses cannot use them without a new electrical service or updated gas supply.
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