A scientist places 25 ml of a yellow substance into a 50-mL container. The substance quickly fills the entire container. Is it a solid, liquid, or gas?
It's a gas.
Gas has the highest entropy, or tendency to disorganize itself when given the opportunity. Solids have the least disorder, and liquids are somewhere in between, but still lean closer towards the lowest entropy.
Consider each of the three. If you "poured" a solid into another container, it is unlikely that you'll see it spontaneously expand to fill the entire volume of the 50mL container.
If you poured a liquid, like water, to the 50mL container, it will still take up roughly the same volume as it initially did. This is to say that the act of pouring the liquid won't be capable of doubling the volume (thus doubling the mass). Liquids are incredibly hard to compress.
The volume of the gas, however, is essentially determined by how much room its given. If it had the chance, it would love to take up the space of a 100mL container as well, though there would be much less pressure in this container than the 50mL container. Removing the gas substance from the 25mL container and transferring it to the larger container will lower the temperature and pressure of the gas, but in doing so it increases the volume.
Pv = nRT