When a person gets engaged and married there are various things that we do as a Zecher L'Churban. At the Tenayim (whether done immediately or at the wedding) we break an earthernware plate, at the Chupah we break a glass. Why do we Davka break an earthernware plate at the Tenayim and a glass at the Chupah?
I heard it is because one can "fix" a wedding through a divorce (like a glass that can be melted back together). There is no way out of a tnaim, however, so it is like an earthenware plate that can't be fixed.