What interrupted Bruno's conversation with his sister?

Bruno's conversation with Gretel is interrupted when she begins to scream that she hs live in her hair...... as does Bruno.

Bruno was interrupted by the sound of Gretel breaking into a piercing scream; one that woke Mother up from her afternoon nap and brought her running into the bedroom to find out which of her children had murdered the other one. While experimenting with her hair Gretel had found a tiny egg, no bigger than the top of a pin. She showed it to Mother, who looked through her hair, pulling strands of it apart quickly, before marching over to Bruno and doing the same thing to him. ‘Oh, I don’t believe it,’ said Mother angrily. ‘I knew something like this would happen in a place like this.’ It turned out that both Gretel and Bruno had lice in their hair,