Remove the tank cover and check the water level after the fill valve has shut off. It should be about one inch below the opening of the overflow tube. If it is significantly lower than that, there may not be enough water in the tank to fill the bowl after a flush. Correct this by adjusting the float.
The average child can do this during the second half of this year. Between 24-36 months, children are typically able to work simple insert puzzles (e.g., completes a three-piece simple puzzle where pieces are whole objects). Also, children can remove a part from a toy (e.g., a wheel) and replace it.
At around the age of eight or nine months, infants are more interested in an object for the object's own sake. A discovery by Piaget surrounding this stage of development, was that when an object is taken from their sight, babies act as though the object has ceased to exist. By around eight to twelve months, infants begin to look for objects hidden, this is what is defined as 'Object Permanence'.