This was an exam question of ours:
Let $\chi$ be the set, $\chi = \left \{ a, b, c, d \right \}$. Create a topology $\tau$ on $\chi$ so that $\left ( \chi ,\tau \right )$ is regular but not normal.
I couldn't solve the question but I'm wondering what the answer is. Is it possible at all? Can anyone help me with this?