A simple starter to prepare – just search for an image (maybe topical) and display it on the board, with the question: “What’s this lesson about?”
It’s up to you how obvious/cryptic you make the image, but whatever you do it’ll get the kids thinking and you may be surprised with what they come up with. I once showed an image from a news story showing a grounded ship in the background and people hauling spilled cargo from it up the beach – I thought it was pretty obvious but it took a while before someone realised the lesson was about floating and sinking…
Word of warning – if you’re using some sort of interactive whiteboard software or powerpoint, the filename is often displayed somewhere on the screen. So if you save your presentation as floating-and-sinking.notebook and try this starter, they may get it quicker than you expected!