As what has been described as a “pitched battle” between anti and pro Mubarak protestors rages in Cairo’s Tahir square, reports continue to trickle out that both sides have used a weapon as effective now as it was when it was invented more than 70 years ago.

The Molotov Cocktail, so named by Finns who used it against Soviet forces in the Winter War of 1939, is a remarkably powerful weapon, for one simple reason: the astonishing energy density of gasoline.

The amount of energy stored in a given volume of gasoline is 36 times higher than a lithium ion battery, 15 times that of gunpowder and 10 times greater than the energy per unit volume of TNT.

Molotov cocktails were so effective when first invented that they were capable of taking out tanks. (Can you think of anything else in your home that could even come close?) Incredibly, half a century later, that’s still how they’re being used, according to this remarkable footage of a Molotov attack on an Egyptian army tank in Cairo.

Even as the tables have turned on anti-Mubarak protestors in Tahir square, at least one report coming out of Cairo holds that army tanks were rolling out of the way so as not to be hit by Molotov cocktails thrown by protestors. Modern tanks can’t be harmed by Molotov cocktails, but their sensor can be blinded, and they appear to have a powerful psychological effect on the soldiers inside.

But again, it’s not the delivery mechanism that matters, it’s the gasoline inside that is such a powerful equalizer when used by citizens who are otherwise unable to access weapons on par with those wielded by modern militaries. The effectiveness of a Molotov Cocktail derives from the same underlying physics that dictates that that all-electric vehicles and energy-dense batteries are signifiant engineering challenges, if only because the energy density of liquid fossil fuels is so difficult to match. As an energy storage mechanism, they are unique, and Molotov cocktails are yet another tiny, visceral reminder of that.