A teaspoon of material from a neutron star would weigh a billion tonnes on Earth.
Why it is true
Neutron stars are the ultra-dense remnants of stars that exploded. Their matter is so compressed that a teaspoon would have a mass of about a billion tonnes. This happens because protons and electrons combine to form neutrons, eliminating the empty space between them.
Source: NASA
Would you rather be asked first?
This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.