The more people who see an emergency, the less likely any one of them is to help.
In 1968 John Darley and Bibb Latané found that people who believed others could also help were slower to act in a staged emergency. Responsibility feels shared — and diluted.
Why it matters: In an emergency, point to one person and ask them directly for help.
Bystander intervention in emergencies: diffusion of responsibility — John Darley and Bibb Latané (1968) Journal of Personality and Social Psychology, 1968
A signal before mealtime taught dogs to expect food.
Studying digestion, Ivan Pavlov noticed dogs began to salivate at signals that came before food. Repeated pairing taught them to respond to the signal alone — classical conditioning.
Why it matters: Many habits and fears are learned this way — which means they can be unlearned too.
Conditioned Reflexes — Ivan Pavlov (1927) English translation, 1927
A famous test of patience turned out to be more complicated.
In Walter Mischel's Stanford studies around 1970, children who waited longer for a second treat later did better on some measures. A larger 2018 study found the link was much smaller once family background was considered.
Why it matters: Good science updates its conclusions when better evidence arrives.
Delay of gratification studies — Walter Mischel; Tyler Watts, Greg Duncan and Haonan Quan (1972; 2018) Psychological Science, 2018 (replication)
In 1929 Edwin Hubble showed that distant galaxies are moving away from us, and the farther away they are, the faster they recede — evidence that the universe is expanding.
Why it matters: This discovery led to the Big Bang theory of how the universe began.
A Relation between Distance and Radial Velocity among Extra-Galactic Nebulae — Edwin Hubble (1929) Proceedings of the National Academy of Sciences, vol. 15
Moving the Sun to the centre changed how we see our place in the universe.
Copernicus's book, published in 1543, argued that Earth and the planets orbit the Sun. Galileo's observations and Kepler's laws later supplied the evidence and the mathematics.
Why it matters: It showed that common sense can mislead us — careful observation must decide.
De revolutionibus orbium coelestium — Nicolaus Copernicus (1543)
A thousand years ago, a scholar measured the Earth from a hill near Jhelum.
At Nandana Fort, al-Biruni measured how far the horizon dipped from a hilltop and used trigonometry to estimate Earth's radius at about 6,340 km — close to today's 6,371 km.
Why it matters: With simple tools and clear reasoning, he came within about half of one per cent.
Tahdid Nihayat al-Amakin (Determination of the Coordinates of Places) — Abu Rayhan al-Biruni (c. 1025)
There is no largest prime number — and the proof is about 2,300 years old.
Euclid showed that if you multiply any list of primes together and add one, the result has a prime factor that is not on your list. So the primes never end.
Why it matters: Large primes now help keep online banking secure through encryption.
Elements — Euclid (c. 300 BCE) Book IX, Proposition 20
Zero is a number — and someone had to write down its rules.
In 628 CE the astronomer Brahmagupta gave rules for calculating with zero, such as a number minus itself is zero. These numerals later reached Europe through Arab scholars.
Why it matters: Our place-value number system, and all digital computing, depends on zero.
The words "algebra" and "algorithm" both come from one Baghdad scholar.
Around 820 CE, al-Khwarizmi wrote a book on solving equations by al-jabr (restoring) and balancing. Algebra is named after it, and "algorithm" comes from his own name.
Why it matters: Every computer program is an algorithm — a step-by-step method like the ones he wrote down.
The Compendious Book on Calculation by Completion and Balancing — Muhammad ibn Musa al-Khwarizmi (c. 820)
A Dutch cloth merchant was the first person to see bacteria.
Antonie van Leeuwenhoek ground his own powerful lenses and in the 1670s described tiny living "animalcules" in water and scrapings from teeth, in letters to the Royal Society.
Why it matters: Microbiology — and later germ theory and antibiotics — began with his curiosity.
Letters to the Royal Society — Antonie van Leeuwenhoek (1676) Letter of 9 October 1676
Looking at cork through his microscope, Robert Hooke saw tiny box-like spaces that reminded him of small rooms, and called them cells. He published his drawings in 1665.
Why it matters: We now know every living thing is made of cells — the starting point of biology and medicine.
Micrographia — Robert Hooke (1665) Observation XVIII
The instructions for every living thing are written in a twisted ladder.
In 1953 Watson and Crick published the double-helix structure of DNA, built on X-ray images by Rosalind Franklin and Raymond Gosling, including the famous Photo 51.
Why it matters: Understanding DNA led to genetic testing, new medicines and forensic science.
Molecular Structure of Nucleic Acids — James Watson and Francis Crick (1953) Nature, vol. 171, 25 April 1953