If ordinary water molecules can contain any of the three hydrogen isotopes, $\ce{^1H}$, $\ce{^2H}$, $\ce{^3H}$, and any of the three oxygen isotopes, $\ce{^{16}O}$, $\ce{^{17}O}$, $\ce{^{18}O}$, how many kinds of water molecules $\ce{H2O}$ are there?
I'm a little confused because since there are $\binom{3}{2}$ ways of choosing two hydrogen atoms and 3 ways of choosing oxygen atoms, shouldn't there be 9 different kinds of water molecule? Why would it be 18 (as the answer says)? In other words, why would the order of hydrogen atoms matter here?