the thing cannot be said

In an island somewhere in the Pacific, a mother and daughter were checking in at the immigration and had to go through the standard screening.

Daughter: What are they doing, mum?
Mum: They are checking for things?
Daughter: What things mum?
Mum: Hmmmm...that thing?
Daughter: What is that thing?
Mum: Officer, can you please tell my little girl what you are checking?
Officer: Getting annoyed, 'We