In most action movies, explosions are just a part of daily life. Your typical hero can hardly walk down the street without dodging exploding bombs, cars and gasoline tankers.

Assuming you had this kind of bad luck, what would the explosions really be like? Here's the true story.

Explosions 101
Simply put, an explosive is something that can burn very rapidly. Burning is caused by a chemical reaction between oxygen and some kind of fuel (like gasoline or C-4 plastic explosive). Generally, heat or a physical blow triggers the reaction.

The burning process releases various gases. Explosive material burns very quickly, so it releases a huge amount of gas in a small amount of time. All of this concentrated gas rapidly expands to fill the available space, applying a lot of pressure on everything nearby. This is the actual explosion: expanding gas that is powerful enough to destroy cars, buildings, or whatever else is around.

The reaction also releases a lot of heat, which means the atoms in the air are excited. And depending on the particular explosive material involved, the excited atoms can emit light (in other words, you can see a ball of fire).

Can cars blow up in big fiery explosions when they crash, like in movie car chases?
Well, they can, but it's not very likely, mainly because the car's gas tank would have to rupture. Most cars have extremely durable tanks, surrounded by crumple zones and other protective features, so that doesn't happen often.

Assuming the tank did rupture, the liquid gas would need to spread out over a large area, change form into gas vapor (only gasoline in vapor form will burn), and mix with the air in the right proportions. Then you would need some ignition source, like a spark, to set the explosion off.

All of that could certainly happen, but it would take a little while. It probably wouldn't happen at the instant of impact.

What about a gasoline tanker truck?
Tanker trucks, on the other hand, can and do explode - with big fireballs, no less. You can't reinforce this kind of tank the same way you can reinforce a car's gas tank, because the truck would be way too heavy. That means it's a lot more vulnerable -- if a tanker truck gets into an accident, the tank may break, spilling gasoline everywhere (and a typical tanker can hold 10,000 gallons of gasoline). Sparks from the truck dragging on asphalt can ignite the vapor and set the whole thing off.

Can you really outrun an explosion, like Sydney does every other week on "Alias"?
Let's go to the starting blocks: Sydney vs. chunk of C-4 explosive.

Let's say Sydney can sprint at about 15 miles per hour, or 22 feet per second. If we give her a 10-foot head start to get to top speed, she'll be 32 feet away from the center of the blast one second after the C-4 detonates. Not too shabby, but it's not nearly fast enough. A C-4 explosion will expand at a rate of 26,400 feet per second. In other words, the blast is so fast it's almost instantaneous. If she were in range of the explosion, she wouldn't have time to think about running - or anything else.