Why does Moyna react so angrily when Fokir gives a sting-ray tail to Tutul to play with?

Moyna reacts so angrily when Fokir gives a sting-ray tail to Tutul to play with because she is mad that Fokir has taken Tutul on his boat with him so much instead of Tutul going to school. Moyna is generally angry with Fokir for his lack of ambition and drive.