How many students know the chain rule, let's say transforming from (x,t) to (z, tau)
z = z(x,t) = x/s(t)
tau = t
Now transform heat equation
u_t = u_xx for 0 < x < s(t)
to a PDE in z and tau for 0 < z < 1.
// 95% get it wrong first time.
Hint; use the chain rule and z is a function of TWO variables.