Five Science Facts That Sound Fake but Are True

Every fact below reads like something from a dubious listicle. Every one of them survives contact with the primary literature. Here are five, with the real numbers.

1. Bananas are radioactive

Bananas contain potassium, and about 0.012% of natural potassium is the radioactive isotope potassium-40. A typical banana carries roughly 420 mg of potassium, whose K-40 decays about 14 times per second — around 14 becquerels of activity. That delivers about 0.1 microsieverts per banana: less than 1% of your daily background radiation, which itself comes mostly from radon and cosmic rays. Your own body, full of potassium, is already emitting about 4,000–5,000 becquerels. Bananas are not dangerous — they’re just measurable.

2. Octopuses have three hearts and blue blood

Two branchial hearts pump blood through the gills; a third systemic heart drives it to the rest of the body. Oddly, the systemic heart stops beating while the octopus swims — one reason they prefer crawling. Their blood runs blue because it uses hemocyanin, a copper-based oxygen transporter (our iron-based hemoglobin is red). The blue is only visible when oxygenated; deoxygenated octopus blood is clear-ish.

3. There are more possible chess games than atoms in the observable universe

Claude Shannon’s 1950 estimate put the number of possible chess games at roughly 10^120 — the “Shannon number,” and it’s a conservative lower bound. The observable universe contains on the order of 10^80 atoms. That’s a gap of about 40 orders of magnitude, so vast that better estimates on either side don’t change the conclusion. For scale: 10^120 is a googol times a googol times a hundred.

4. A day on Venus is longer than its year

Venus takes 243 Earth days to spin once on its axis — and it spins backwards (retrograde). It takes only 225 Earth days to orbit the Sun. So the sidereal day outlasts the year. Because of the retrograde spin and the orbital motion, a sunrise-to-sunrise day on Venus is about 117 Earth days — still roughly half its year, and the longest solar day in the solar system.

5. Your body has roughly as many bacteria as human cells — not 10x more

The old “10 times more bacteria than human cells” claim is a myth. A widely cited 2016 estimate (Sender, Fuchs & Milo, Cell) put the numbers at about 3.9 × 10^13 bacterial cells versus 3.0 × 10^13 human cells — a ratio near 1.3:1, dominated by the bacteria packed in your colon, weighing roughly 0.2 kg in total. The ratio varies person to person and the real lesson is humbler and more interesting: you are roughly half a community.

The common thread: the accurate version of a wild-sounding fact is almost always stranger — and better — than the meme that popularized it.