Tasks such as these are usual in classrooms.
Colour two thirds of this shape.

What fraction of this shape is shaded?

Students learn to connect a partwhole area diagram with the written fraction by thinking, "The denominator tells how many parts, and the numerator tells how many are shaded."
While this is a useful basic strategy, it is problematic for several reasons.
 The written fraction is seen as a pair of whole numbers, each recording the count of something different: a 'double count'.
 Many students believe this double count to be the actual definition of a fraction.
 A fraction is not comprehended as a single number, having a value like other numbers.
 The double count cannot be generalised to all situations, such as "Share two pancakes equally among four people."
 Students can lose sight of the whole and interpret a partwhole situation as a ratio, especially when working with fractions of collections.