Write on the board the topic of your lesson. Write "[YOUR TOPIC] is like a ____." Give your students a scrap piece of paper and a pen or pencil. Set the timer for 15-30 seconds and let them write their answers. Give them a few minutes to share and explain their answers.
When I did this, I wrote "Prayer is like a ____." and I had the students write their answers on a Styrofoam plate. When time was up, they displayed their answers all at once, and we discussed them. You will get all sorts of responses from good to wacky. One student tried to throw me with "apple," because one a day keeps Satan away. 🙂 I used his metaphor throughout the lesson. Each student described why they chose their word. It helped us start a good discussion on prayer.