At 7:17 in the morning on June 30th, 1908, a huge explosion occurred several miles in the air above the Tunguska region of Siberia, with the force of a large nuclear blast. It was probably a small asteroid or comet. The extreme heat generated by the object entering our atmosphere caused it to violently disintegrate before hitting the ground. Witnesses hundreds of miles away reported that it was brighter than the Sun. Even at that distance, the shockwave knocked people down, ruining their breakfast. The explosion destroyed 60 million trees.

Nineteen years elapsed before Russian scientists got around to investigating the site of the impact. This was partially due to the extreme remoteness of Siberia, but also because of a long term, highly coordinated mass procrastination experiment that was already in progress.

The scientists persuaded the government to fund the expedition, based on the assumption that iron from the meteorite could be retrieved and used by Soviet industry. Because the explosion occurred in mid air, no meteorite was found, but the scientists were able to bring back large quantities of fresh Tunguska, which is quite delicious on toast.

Sleep well, my friend.

In Gulliver's Travels Jonathan Swift described the two moons of Mars, Phobos and Deimos, giving their exact size and speeds of rotation. He did this more than 100 years before either moon was discovered.

At a jet plane's speed of 1,000 km (620mi) per hour, the length of the plane becomes one atom shorter than its original length.

In the Durango desert, in Mexico, there's a creepy spot called the "Zone of Silence." You can't pick up clear TV or radio signals. And locals say fireballs sometimes appear in the sky.

The number pi is the ratio between the circumference of a circle and its diameter. It's approximately equal to 3.14159265, although the digits go on forever.

Some mathematicians are obsessed with computing pi to more and more digits. In the year 1610, a German mathematician computed pi to 35 digits. In 1789, a Slovene mathematician computed pi to 140 digits. This was all done by hand, in poorly heated houses.

An English amateur mathematician spent 20 years calculating pi to 707 digits, finishing in 1873. 71 years later, it was discovered that he had made a mistake at the 528th digit, and all the digits following it were wrong.

In 2002, frantic Japanese mathematicians used a supercomputer to accurately compute pi to 1,241,100,000,000 digits.

Based on all this effort, you might assume that it'd be useful to know a trillion digits of pi. However, if you had a circle the size of the observable universe, and you wanted to compute its circumference with an accuracy equal to the size of a proton, the number of digits of pi that you'd need is only 43.

Either mathematicians are totally crazy, or they're planning ahead for a time when the survival of humanity will depend on the ability to construct extremely large, accurate circles.

The force of gravity seems like a big deal when you have to get out of bed in the morning, or you're helping a friend move furniture around, or when somebody drops a bucket of rocks and gravy out of a window onto your head. However, compared to the other forces, gravity is pretty weak.

There are four known forces in the Universe. Two of them, the strong and weak nuclear forces, keep atoms from flying apart and stuff like that. You shouldn't be concerned about them unless you are a nuclear physicist. If anybody asks, you didn't hear about them from me.

The other two forces are electromagnetism and gravity. If you slam your hand on a table, electromagnetism is what keeps your hand from flying through the table into your lap. This force is widely credited with the success of the restaurant industry.

Gravity pulls you toward the center of the Earth, and keeps you from drifting up toward the ceiling and getting it dirty.

Incidentally, Deimos, one of the two moons of Mars, is so small ("only" two million billion kilograms), and consequently has such weak gravity, that if you were standing on it, and you jumped hard, you could easily achieve escape velocity and never land back down on the ground.

Electromagnetism is also generated by magnets, and causes metal objects to be attracted to each other. A hand-held magnet is strong enough to lift a small screwdriver off a table.

Think about what's happening with gravity in this situation. The electromagnetic force generated by the magnet is strong enough to lift the screwdriver up off the table, overcoming the force of gravity. But what's generating the gravitational force?

The whole Earth!

The Earth has a mass of six million billion billion kilograms, and yet its gravity isn't enough to overcome the magnetic force generated by a magnet whose mass is a small fraction of one kilogram.

I'm out of shape and I have a desk job, and I can overcome the gravitational force created by the mass of the entire Earth with my little finger. Gravity isn't all that impressive.

Neutrinos are extremely small particles that are produced by nuclear reactions. The crazy thing about them is that they barely interact with normal matter, like automobiles, dentists, or enchiladas. They're so weakly interacting that if you shot one through a lightyear of lead, it would only have a 50% chance of hitting anything along the way. Compare this to photons, which are what light is made out of. They're easily stopped by construction paper (and dentists).

Recently, physicists have theorized that out of all of the elemental particles that exist in the Universe, neutrinos are the most child-safe. Neutrinos also have a reputation for not being very fun at parties.

The nuclear fusion reactions in the Sun generate a huge number of neutrinos that are flying straight through the Earth all the time. The Sun generates so many neutrinos that one hundred billion of them pass through each of your fingernails every second.

Scientists detect neutrinos using tanks filled with fifty thousand tons of water, located one kilometer underground, to avoid interference. In the extremely rare event that one of the neutrinos flying through the tank hits one of the water molecules dead on, sensors on the walls of the tank register a very dim flash of light.

Every second, one billion billion neutrinos fly through the tank. The tank has roughly one hundred million billion billion water molecules in it. Despite the overwhelming odds, it is necessary to hire specially trained, extremely patient scientists to wait around for almost two hours before a single neutrino will hit one of the water molecules. During this period, the scientists will typically play poker or watch embarrassing YouTube videos of other scientists.