Defining when something is wet is surprisingly hard. In fact, there still isn't a commonly agreed upon scientific definition. Despite this, we can describe the general concept to young children.
For example, after dunking a towel in a swimming pool, it's clear that the towel is wet, and if the towel has been left out in the sun for long time it's clear that it's dry. But, what if we took an eyedropper and put a single drop of water on it. Most people would agree that the towel isn't wet at this point. It might be considered wet in an area around where you placed the water, but the towel as a whole wouldn't be considered wet.
While extremes are relatively easy to identify, observing the transition from one state to the other can be hard to clearly articulate.