Water but it's a bit of hit and miss that can turn soggy, better is bicarbonate that triggers are more or less instant reaction (often in baking powder in a pinch, but that's mostly a waste compared to just bicarb).

Often if one wants to make something "larger", dropping superglue, adding bicarb with a silt, blowing away and dropping another layer works fairly well (it's a bit of a brittle but still quite hard mass that is created quickly).