Fill them with different amounts of water and talk about concepts like, full, fuller, empty, heavy, heavier, light, lighter, little and much heavier etc.. (shirleys-preschool-activities.com)
The dough will require different amounts of water on different days - the weather affects this - so don't be afraid to use more if you need to. (inerikaskitchen.com)
It's a function of temperature, so, for example, the same 70 percent relative humidity results from different amounts of water in the air. (drtimball.com)