There are some comets whose discovery has been a huge achievement for astronomers. One of them is Comet 67P/Churyumov-Gerasimenko.
The European Space Agency's (ESA) historic Rosetta Mission flew very close to this comet and even landed a lander named Philae on it!
This comet is the size of a rubber ducky! Its nucleus (main part) is actually made up of two parts—a large 'lobe' and a smaller 'lobe', which are connected by a narrow 'neck'. The longest part is about 4.3 kilometers and the widest part is about 4.1 kilometers. Imagine, about the size of Manhattan Island in New York!
The total volume is about 18.7 cubic kilometers. But its mass is only 1.0x10^{13} kilograms. This makes its average density 533 kg/m$^3$, which is less than half the density of water (1000 kg/m$^3$)! That is, it is made up of ice, dust, and empty space, much like a light sponge.