2

Two is the only even prime, which is why so many results in number theory are stated for odd primes with two handled separately. The exception is structural rather than a curiosity.

Binary won over decimal in computing because a switch has two reliable states and ten does not. Early machines including ENIAC used decimal internally, and the change was an engineering decision about tolerance to noise.

The number of tin cans a string telephone needs is 2. It works by transmitting vibration along the taut string rather than through air, which is the same principle as a mechanical gramophone and the reason the string must not touch anything.

There are more than 2 million rivets in the Eiffel Tower (France). 2.5 million, driven by hand. The tower was built in twenty-six months and was meant to be dismantled after twenty years.

2 is the base of the binary number system, in which every value is written with only 0 and 1. Digital computers use base 2 because two states map directly onto a physical switch being off or on, which is far easier to build and read reliably than a ten-state component would be.

The multiple by which one group must exceed another before a six-month-old reliably notices is about 2 (six-month-old infants; the ratio needed narrows with age). Written as 2:1 — sixteen dots against eight, not sixteen against twelve. The approximate number system compares ratios rather than counts, which is why it is sharp for small differences in small numbers and vague for large ones.

The number of heartbeats in an average human lifetime is roughly 2 billion (adults). Most mammals get a similar total across very different lifespans, which is why a mouse's heart is so fast and a whale's so slow.

There are 2 elements that are liquid at room temperature (worldwide, at 25 degrees Celsius). Mercury and bromine. Caesium, gallium, rubidium and francium melt just above it, which is why the count depends on where room temperature is set.

There are roughly 2 million species of fungus. Estimates run from about 2 to 4 million, against roughly 150,000 formally described. Fungi are mostly underground or inside something else, which is why so few have been found.

There are 2 lungs in the human body, of unequal size (humans). The left lung is smaller, with two lobes rather than three, to make room for the heart.

There are 2 chambers in the heart of a typical fish (most fish).

There are roughly 2 seconds in the span the phonological loop can hold without rehearsal (working-memory research; the figure is a range, not a constant). It is why digit spans are shorter in languages whose number words take longer to say — Welsh speakers score lower than English speakers on the same task for no reason but syllable length.

There are 2 wings on a fly, where most insects have four (Diptera, the true flies). The name Diptera means two wings. The hind pair evolved into halteres — small club-shaped organs that beat rapidly and act as gyroscopes, which is most of why a fly is so hard to swat.

The mass of the Sun is about 2 × 10³⁰ kilograms (the Solar System). It holds about 99.86 per cent of all the mass in the Solar System.

There are roughly 2 trillion galaxies in the observable universe (the observable universe). The estimate is contested and has moved by an order of magnitude in both directions. A 2016 estimate that revised the earlier figure of 200 billion upward tenfold; later work has argued back down again.

The length of the DNA in a single human cell, uncoiled is about 2 metres (humans). Across all the cells in a body the total runs to billions of kilometres, which is a figure that means little except as a demonstration of how tightly it is packed.

There are nearly 2 billion stars mapped by the Gaia space telescope (the Milky Way and nearby galaxies). 1.81 billion in the third data release — the largest and most precise star map made, and still under one per cent of the galaxy.

There are about 2 billion tonnes of steel produced each year. Figure as of 2023. 1.9 billion tonnes, over half of it in China.

The mass of Jupiter is about 2 × 10²⁷ kilograms (the Solar System). 1.9 × 10²⁷ kilograms — more than twice everything else in the Solar System put together, leaving out the Sun.

The distance to the Andromeda galaxy is about 2 million light-years (the Local Group). 2.5 million light-years, and the most distant thing visible to the unaided eye. It is approaching us, and will merge with the Milky Way in about four billion years.

There are about 2 × 10²¹ water molecules in a single drop (worldwide, a 0.05 millilitre drop). 1.5 × 10²¹. More molecules in one drop than there are drops in all the oceans.

2 is the atomic number of helium, an inert noble gas.

There are about 2 billion cups of coffee drunk each day. Coffee is among the most traded agricultural commodities by value.

There are about 2 million books published worldwide each year (worldwide, new titles). Counts differ substantially by what is included — self-published titles, reprints and editions in multiple formats can double the figure. Figure as of 2023.

There are roughly 2 billion people without safely managed drinking water. Figure as of 2022. The measure counts safe management rather than access: a source can exist, be reachable, and still fail on contamination or reliability, and the gap between the two definitions is most of the number.

The number of decimal places most currencies use is 2 (worldwide, ISO 4217). Not all: the Japanese yen and Korean won have none, and the Tunisian dinar has three. Payment systems that assume two have failed on all three.

2 is the only even prime number: every other even number is divisible by 2 as well as by 1 and itself, giving it more than two divisors. This makes 2 the sole exception in every proof and pattern that treats 'odd' as a property of primes.