Re: Why does a spoon in a cup keep the cup from cracking when you add hot water

The spoon is a heat conductor, so that it tends to remove some of the heat from 
the liquid as it is poured over the spoon and removes some of the heat from the 
bottom of the bowl or cup after the bottom becomes heated.  The spoon also 
breaks up the initial flow of hot liquid into the bowl so the bottom of the 
bowl is heated more uniformly.  Other than that, most cooking bowls, canning 
jars, and the like, nowadays, don't seem as sensitive to cracking when adding 
hot liquids.  More importantly, it is not certain any of the contributions of 
the spoon are that significant.

When the 'spoon in the bowl' practice began, it is probable that home heating 
in the winter months was such that the real cause for the breakage was due to 
pouring a near-boiling liquid into a colder (maybe in the range of 40 to 50 
degrees Fahrenheit) glass container.  In that instance, every little bit of 
precaution helps, the surest being warming the bowl or cup before adding the 
hot fluid, though a spoon would certainly help a little and possibly just 
enough to prevent cracking.

References?  You might try a heat transfer textbook, though it would only help 
in understanding how heat might be conducted through the spoon.  From such a 
textbook it would also be possible, after a lot of computer programming work, 
to model heating of the bottom of the bowl when a spoon interrupts the flow.  
Once you have the heat transfer processes in hand, you would need a materials 
science reference, which would be of help in understanding how the glass 
responds to both a rapid change in temperature and to an uneven (different 
parts of the glass heated faster) change in temperature.

Bottom line?  Mom's know best.  I have witnessed the effects of metal spoons 
touched to the center of a boiling pot to calm boiling, and it works like a 
charm, removing a little of the heat from the bottom of the pot and the boiling 
liquid, and possibly causing an alteration in the flow patterns in the boiling 
liquid (another variation on the above, which an engineer would need another 
several texts on fluid dynamics to successfully model.)

Thanks for your question.

sid