In this video you can see how to prepare dental materials for impressions, in this case alginate. In the beginning of the video you can see how you can make a mistake in the preparation.
If you add to much water fast, you get a supstance that is liquid and can't be used to imprint the soft and hard tissues in the mouth. In the second part of the video you can see how you can do everything right- first you take the powder, and then add water slowly and in small amount.This way you get the semi-solid material you need.