Demonstrations are sometimes perceived as merely entertaining and expendable ancillaries for lectures and laboratory sessions. Nothing can be further from the truth. If done properly, demonstrations have much more value than lectures and labs when used to teach critical thinking in the sciences. There are effective ways to model scientific principles in a manner that allows students to visualize, practice, and apply the information being presented. This article describes the properties and format of an effective demonstration, and presents two sample demonstrations. (Contains 2 figures.)