Three rings are locked together, yet no two of them are actually linked to each other
Three circular rings are arranged so that all three together form a single connected, inseparable knot. But look at any two of the three rings on their own, ignoring the third: are they linked to each other, or could you pull that pair apart?
Reveal the answer
Any two of the three rings, taken alone, are completely unlinked and would simply fall apart if the third ring were removed. It's only the specific over-under-over arrangement of all three together that locks them in place, a structure now called the Borromean rings after the Italian Borromeo family, who used it as a heraldic symbol, though the pattern appears far older in Norse and Japanese art. Mathematician Peter Guthrie Tait formally catalogued it within knot theory in 1876.
— Peter Guthrie Tait, On Knots — Transactions of the Royal Society of Edinburgh, 1876