More often than not, students greet that idea with incomprehension or at best with mild fascination. For authentic learning to happen time should occasionally be wasted, tangents pursued, side-shoots followed up. Some of the greatest discoveries have been made through serendipity. So, too, learning moments are occasionally stumbled upon inadvertently. A teacher should feast on these, follow their flow, be humble before them.
Tests are a very poor indicator of whether an individual has really learned something.
The main reason for this is that, except for disciplines which are extremely abstract and theoretical, tests provide an artificial context for demonstrating one's knowledge. They seldom address the real world. Learning first takes place through osmosis. Before anything else it is a process of unconscious immersion in one's immediate environment. Though school learning is more formal and conscious, the osmotic process still goes on.