How did Matilda manage to convince her family that there was a ghost in the house?

The young Matilda gets so angry at her bullying father for ripping up her copy of The Red Pony that she plots revenge. She visits her friend Fred and trades him a week's allowance for the chance to borrow his parrot. The parrot likes to say “Hullo” and “Rattle my bones!”
Matilda hides the parrot in the dining room chimney of her home. When the bird starts to squawk "Hullo" and "Rattle my bones," the family thinks it is burglar. When they search and find no burglar, Matilda says it must be a ghost. This frightens her family into running off. Later, they return, and her parents are nicer to her for about a week.
The Wormwoods are incompetent parents who Matilda can easily outsmart. Matilda is young, but she is more intelligent than they are, as is illustrated by this story of how easily she convinces them the house is haunted by a ghost.