बालकों के संज्ञानात्मक विकास (Cognitive development) के क्षेत्र में अग्रणी योगदान देने के लिए किस मनोवैज्ञानिक को सार्वभौमिक रूप से जाना जाता है?

What skill can be developed by asking children to arrange pebbles according to categories like shape, size, or colour?

In Jean Piaget's cognitive development theory, if a child encounters a completely new food and cannot fit it into their existing schema, what cognitive state do they experience?