The science behind a Prince Rupert’s Drop is pretty neat, actually. I’ll try to lay it out for everyone, and keep science jargon to a minimum.

Okay, for starters, the glass in the drop is considered a tempered glass. In fact, it might well be the first tempered glass material in history, dating back to the 1600’s. A tempered glass behaves like a composite material – a good example of which is concrete. Composite materials can cope and deal with extreme compressive forces (e.g. cars driving on top of it, people walking/stomping, general “push” forces) as well as deal with tensile forces (“pulling” forces) quite well. Composite materials, however, do no deal with shear forces (cutting/twisting, etc) very well at all.

That said, when glass is liquefied and rapidly cooled (like in the video), the outer surface of the drop begins to solidify very rapidly. As it does, it begins to contract and become more dense, and the outer surface begins to exert compression on itself. However, while the outer surface is solidifying very quickly, the inner core cools much more slowly and solidifies at a much slower rate. What begins to happen is the outer surface (the shell, so to speak) solidifies so fast that the inner core (the softer glass) is pulled outwards as the shell contracts on itself. The result is that the outer surface of the drop is under a great deal of compression, while the inner core is under a great deal of tension.

So when you bang on the drop with a hammer, stomp on it, etc, you’re just converting all of the tensile forces inside of the drop into compressive forces – you’re actually making it MORE able to resist breakage from compression. Fun, eh?

However, when you damage the tail (I hear all you have to do is scratch it), you exert a SHEAR force on the drop. When this happens, it alters the compression on the surface of the drop – in fact, it actually removes the compression on the surface of the drop. When this happens, the tension inside of the drop pushes the walls outward so quickly that the drop explodes.

According to that Wikipedia link, the “crack front” propagates across the surface of the drop at a rate of about 4,200 miles per hour. To put that in perspective, the speed of sound in dry air is about 770 miles per hour.

I have no idea about the science behind this, but I really really really want to make one.  I've been digging around youtube for a video of it exploding at ultra ultra slow motion

Fixed da vid for you.  "Input format" drop down let's people put in full html.