Niamh,
would they be using their understanding of ‘good’ drinks for teeth to predict what happens to the egg? If they are only guessing with their predictions, then I would use the egg and liquids as the prompt, and then extend their learning once they have seen the effects.
They could wonder – will the egg break down more if we leave it longer? Or, will another type of juice that tastes tangier make the egg break down faster?
Then they are applying their understanding and testing out their ideas.