Checking

It was homework time in the evening and I was looking through Zern's Math homework book.

Me: Zern, you see the note from your teacher here? You've been making careless mistakes.

Zern: Yeah...

Me: You should check your work after you do it and before handing in.

Zern: Oh...

Then, I went blah blah blah to explain each mistake and how he could have avoided it.

As I set about preparing dinner, I left instructions for him to complete his next Math homework and to check it as well. Twenty minutes later, Zern was still looking glumly at his homework sheet.

Me: So, did you check your work after you've done it?

Zern: Why should I?

Me: Why not? How else can you avoid careless mistakes?

Zern (almost teary): But why? I thought this is the teacher's job.

Me: ......

Can someone please help me fill in the blanks?