Why is it that when you boil water...?

Question

I have a friend that boils water and places a knife over the edge of the pot This seems to stop the bubbles from bubbling over. Why is that?

Answer 1

Luck, initially, and unconscious observation after that. Over time, he has been conditioning himself, and now knows when to stop filling the pot with water to prevent an eventual boil over. He unconsciously knows through experience how much rice/macaroni/pasta to put in the pot of water he has filled (usually filled by number - "1 mississippi, 2 mississippi, etc) - Whatever!. That's it! Next time he does this, without warning, remove the knife - I guarantee that, if he filled the pot, if he put the pasta or whatever in it, and placed the knife over the opening, you removing it will not result in an overflow.

It is very similar to parents who say their kids were "toilet trained at 9 months!" no way - they just learned the signs and signals. Try to convince them of that, though! It's easier to prove this with pasta, however, but you must say and do nothing until HE puts the knife over the boiling water. Then remove it. Ego deflated.

Magic (and bogus science) dispelled! Unless you think this is an important and necessary boost to his ego. If so, leave the knife alone and continue to be amazed!

Answer 2

water will never boil over the top of a pot, The Knife deal is fairytale, boiling water is not going to expand any more than the bubbles, and if you are boiling it, it will evaporate and still be contained by the pot

Answer 3

When the bubbles do touch the metal, it causes an enzymic reaction that forces the ions in the water to reverse. The water has no choice but to subside.

Answer 4

Wow, perhaps that is because the knife works as a heat sink.

Answer 5

wow, that's a good questions... is it true though?