What you Need
What to do
Part of the reason that chocolate is so irresistible is the way that it is made up of a variety of fats, both from the cocoa bean and in many cases milk or even vegetable. These fats happen to melt at just below body temperature so the chocolate melts in your mouth. This is why chocolate is so obviously a bad material for making teapots, hence the phrase "as useless as a chocolate teapot"!
So we got several short pieces of perspex tubing and poured various amounts of molten chocolate into the bottom to form a plug of chocolate in the bottom of each tube. We then added boiling water of a similar depth to a teapot. We then waited to see when the chocolate plugs would fail.
As we had no idea how thick the chocolate would have to be, we made a series of chocolate plugs ranging from 10mm to 80mm. We also used dark chocolate to give the teapot the best chance possible, as this has a higher melting point than milk or white chocolate. We also left all the chocolate in the fridge until a few minutes before the experiment.
As it turned out only the 10mm thickness of chocolate failed and even that took about 3 minutes - almost as long as it takes to brew some tea! Armed with this knowledge we set out to manufacture a chocolate teapot.
Manufacturing the teapot
As 10mm of chocolate wasn't quite enough to brew tea properly and a real teapot is larger than our model and so would undergo larger forces, we decided to make the walls approximately 20mm thick. The teapot was made by melting down about 1.3kg of chocolate, pouring some into the bottom of a bowl and allowing it to set to form a base. We then put a smaller bowl in the middle to form an inner mould and pouring chocolate in between the two bowls. Removing the bowls was quite easy once you heated them up with boiling water.
We then made a spout using two cylinders of grease proof paper which chocolate generously splurged in between. We then drilled a hole into the side of the 'bowl' using a potato peeler and welded the spout back on using some more molten chocolate.
We also made a handle using more chocolate and welded that on, though this was more for appearance rather than any real practical purpose! We then used the leftover chocolate to make a rather insubstantial lid, to complete the appearance of a genuine teapot.
The moment of truth then arrived and we tried to brew a pot of tea, so we put in a couple of (earl grey) teabags, added some boiling water, put on the lid, and waited...
What may happen
After only about half a minute the lid started to melt, but otherwise the teapot survived its experience in one piece if not entirely unscathed. The tea was slightly unusual and sweet, but not unpleasant.
The teapot survived, but not completely unscathed...
So a chocolate teapot is really not very useful, but slightly more so than you would expect, which is an interesting reinterpretation of the old adage!
Now all we have to do is work out what to do with 1.3kg of chocolate, and enjoy our tea!
Why does it happen?
When chocolate melts it doesn't become totally liquid immediately, it remains quite viscous. Unless you apply a fairly large force to the melted chocolate, it seems to sit there. Chocolate is also mostly made of fat, which is a good thermal insulator (whales use blubber as a form of insulation). This means that the molten chocolate near the hot water protects the less molten chocolate below it, insulating it from the heat of the water.
Also, it takes a significant amount of energy to melt chocolate, so it will take a significant amount of time to move heat into the solid chocolate, thus slowing its melting.