Presenter notes: Bring in real objects rather than starting at the board: a matchbox, a dice, a tin, a party hat, a ball, a chalk box. Pass them round and ask the class to sort them into groups by any rule they like, without telling them what the rule should be. Most groups will separate the pointy ones from the straight ones and the round ones without any prompting, which is exactly the prism, pyramid and curved split you are about to name. Starting with their sorting and then supplying the words makes the vocabulary feel earned.