A sugar cube of neutron star material would weigh hundreds of millions of tonnes.
A neutron star packs more mass than the Sun into a ball about 20 kilometres across. Its material is so dense that a single cubic centimetre would weigh about as much as a mountain.
Why is this true?
Neutron stars are the collapsed cores of giant stars. Gravity crushes protons and electrons together into neutrons, and the whole star is held up only by the fact that neutrons refuse to be squeezed closer.
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More about space
Space facts are mostly a lesson in scale. Days longer than years, storms wider than Earth, light that left its star before the pyramids existed — the numbers stop being intuitive very quickly. Every entry here comes from established astronomy and includes a short explanation of what causes the effect, so the fact holds up when someone asks the obvious follow-up question at the dinner table.