Why they are "just playing"...

When they are playing with sand, they are learning...
Math concepts: conservation, weight distribution, volume and mass, measuring, geometry, numbers.
Science: exploring, observing, discovering.
Language: comparative terms, requests for information, dialogue, names for things.
Symbolic Representation: using sand in a make-believe way to represent abstract thinking and visualization.
Social Skills: sharing, cooperating, problem solving.

When they are playing with water, they are learning...

Math concepts: fractions, volume, weight, equality differences, measurement.
Science: water as a power source, water conservation, water pollution, water management, evaporation, water cycle, properties of water and objects in water.
Language: vocabulary, phrases.
Social Skills: manners, sharing.
Art: food, paint color in water, mixing colors.
Motor Skills: pouring and coordinated movement.

When they are playing with manipulatives, they are learning...

Math Concepts: patters, ordering and sequence, number, measurement, geometry, algebra, probability, logic.
Representation: using manipulatives to represent solutions, problems.
Self-direction: making choices, making a plan and implementing it.
Scientific Method: observing, making and testing predictions.
Social Studies: using available resources for building, representing structures.

When they are engaged in dramatic play, they are learning...

Language: changing roles, creating a script.
Representation: using language and objects to represent situations in life.
Problem Solving: human interaction, causing others to play cooperatively.

When they are playing with blocks, they are learning...

Math / Science: size, shapes, weight, sorting, patterning, cause and effect.
Reasoning Skills: problem solving, planning.
Social Skills: cooperation, negotiation, conflict resolution.
Literacy: symbolic representation.

When they are engaged in art, they are learning...
Representation: using art materials to represent learning from other curricular areas.
Creative Expression: using color, line form, texture, shape to express themselves.
Problem Solving: using available materials to create, using "mistakes" creatively.
Language: vocabulary, descriptive words.
Cause and Effect: creating change.
Task Completion: staying with a task and experiencing the feeling of satisfaction.