Bloom's Taxonomy of Educational Objectives explains that the process of thinking actually involves several levels. Infants and toddlers use mostly the first two levels, but by age 3 children can use all six.

1. Gathering knowledge consists of acquiring basic pieces of information. Asking children to identify and describe objects encourages thinking on this level.

2. Comprehending and confirming involves looking at the meaning of the knowledge that has been gathered and drawing conclusions from it. A good question to encourage this level of thinking might be, for example, "The yellow sponge floats. What about the other sponges?"

3. Applying entails using what has been learned in new situations. Asking children to consider a newly learned fact as they build or make something can foster this level of thinking. 

4. Analyzing involves thinking about a whole in terms of its various parts. You can encourage this level of thinking by asking children what materials could be used for a particular classroom project.

5. Synthesizing consists of putting parts together to form a whole. Asking children how to use an array of materials to create something, for example, invites thinking on this level. 

6. Evaluating entails making comparisons and judgments. You can encourage this level of thinking by asking children which of the materials they used worked the best.To read more about these six levels of thinking, see Taxonomy of Educational Objectives by Benjamin S. Bloom (Longman).