It's the end of a school day at Coal Hill School. Ian knocks on Barbara's door. Barbara: Come in. Ian comes in and sits down. Ian: I've come to talk to you about Susan Foreman. She hasn't handed in her homework for three weeks. And then makes silly mistakes in science. I was setting the class a sum with A, B and C as three dimensions. Flashback: Susan: You have to use D, E and F, Mr. Chesterton! Ian: Why? Susan: Without it, you can't do the sum! Ian (sighs): Aah. Time as the fourth dimension, I presume. What are the fifth and sixth dimensions? End of flashback. Ian: That excuse is just too good.