Error of Average

A teacher is planning to cross the river with his students. 

He asks the students to calculate average of river depth to know how deep is this river. 

After assessment, it is found that the river is 3.5 feet deep on average.  

Teacher tells none of us is less than 5 feet, so we can cross easily this river. 

Result is that all of them including teacher gets sinked in river because the river is not only 3.5 feet everywhere. 

So average is the biggest deception.