Sometimes, a substance - let's say "Woman" - that in normal circumstances would be frozen, can become a liquid when it is in close proximity to another solid substance - let's say "man". While neither would be fluid without considerable heating when alone, together they become quite volatile. Melting point can be suppressed and boiling point enhanced, but only with this particular combination.

other combinations just don't seem to have the right chemistry...

Next week, in crystallography, we'll review solid solutions - where one element can substitute for another (within certain limits) and mimic it's behaviour and crystal structure.

Later, we'll cover isotopes, where the mimicry is almost perfect - sort of identical-twin territory... (but one is heavier than the other, and only one isotope can mediate a fusion reaction)

8-)