The sparklers I could understand. Ditto the firecracker. But the others...how in the hell does pouring liquid nitrogen into a bottle and putting it in a watermelon cause said melon to blow up??

There's been no explanation as to what it is in Mentos that causes Coke to go ballistic. Someone care to explain??

I don't know why liquid nitrogen causes a watermellon to explode, but as for the Mentos and coke, here's an explanation from Steve Spangler Science:

As you probably know, soda pop is basically sugar (or diet sweetener), flavoring, water and preservatives. The thing that makes soda bubbly is invisible carbon dioxide gas, which is pumped into bottles at the bottling factory using tons of pressure. Until you open the bottle and pour a glass of soda, the gas mostly stays suspended in the liquid and cannot expand to form more bubbles, which gases naturally do. But there's more...

If you shake the bottle and then open it, the gas is released from the protective hold of the water molecules and escapes with a whoosh, taking some of the soda along with it. What other ways can you cause the gas to escape? Just drop something into a glass of soda and notice how bubbles immediately form on the surface of the object. For example, adding salt to soda causes it to foam up because thousands of little bubbles form on the surface of each grain of salt.

Many scientists, including Lee Marek, claim that the Mentos phenomenon is a physical reaction, not a chemical one. Water molecules strongly attract each other, linking together to form a tight mesh around each bubble of carbon dioxide gas in the soda. In order to form a new bubble, or even to expand a bubble that has already formed, water molecules must push away from each other. It takes extra energy to break this "surface tension." In other words, water "resists" the expansion of bubbles in the soda.

When you drop the Mentos into the soda, the gelatin and gum arabic from the dissolving candy break the surface tension. This disrupts the water mesh, so that it takes less work to expand and form new bubbles. Each Mentos candy has thousands of tiny pits all over the surface. These tiny pits are called nucleation sites - perfect places for carbon dioxide bubbles to form. As soon as the Mentos hit the soda, bubbles form all over the surface of the candy. Couple this with the fact that the Mentos candies are heavy and sink to the bottom of the bottle and you've got a double-whammy. When all this gas is released, it literally pushes all of the liquid up and out of the bottle in an incredible soda blast. You can see a similar effect when cooking potatoes or pasta are lowered into a pot of boiling water. The water will sometimes boil over because organic materials that leach out of the cooking potatoes or pasta disrupt the tight mesh of water molecules at the surface of the water, making it easier for bubbles and foam to form.

http://www.stevespanglerscience.com/experiment/109

Basically, it's all about the gas, or at least in this case stuffing that gas into a bottle (watch carefully, and you'll see the watermellonistas put the dry ice or liquid nitrogen into a bottle, and then the bottle goes into the mellon). The volume occupied by a given amount of a liquid or solid increases several hundred times when it becomes a gas. If you do that in a sealed bottle, it becomes pressure, and when that pressure reaches the maximum limit of the bottle it bursts...violently. Pretty simple, eh. Oh, and yeah, those exploding bottles do explode with enough force to cause injury. So be careful if you try this at home, particularly since it's almost impossible to know when it's going to go off.

In this case a typical PET bottle will burst at about 12 atm (about 1250 hPa). That pressure can be achieved with about 25 cc of liquid nitrogen or 30 grams of dry ice in a 2 L bottle. I guess plastic bottles do not generate shrapnel when they burst, but I sure wouldn't want to be holding that bottle when one does.

As for the Mentos in Coke...It has nothing to do with either mentos or coke. The CO2 that's dissolved in the water of any carbonated drink (beer included) can be driven out by adding something else to that. Most likely the candy shell provides nice sites for CO2 nucleation (bubble formation), and that increased nucleation causes the CO2 to rapidly degas from the water. More-or-less, it's like shaking a bottle of soda, with your thumb over the mouth.

I've been wondering about this for a while. Thanks for the information!