Elaine,
would the children’s predictions at the beginning be based on anything more than a guess? If so, then this would make an excellent prompt – and they could be asked after a day what they think would happen the next day. In this way, their understanding is being tested, rather than them guessing and finding out.
One child might think: the gummy bear started small and it got big because it took in water, but it is like my stomach – there is a point when you can’t put any more in, so it won’t get any bigger.
Another might think: the gummy bear got bigger because the water went into it. The outside of the bear can still get bigger, so it will still get even bigger.
Another might think: the gummy bear is mixing with the water, so in another day the gummy bear will be sludge in the glass.
These three different predictions are based on their understanding and have testable results, which you then do over the next day so that they can share their results.
If a child has another question – I wonder if this would be the same in milk? (they might say that milk is mostly water, so yes) – then that would be a testable take the next step.
Here is an example of a home group that tried lots of different liquids.