Had Had
Every kid in the school should know what it is because it's the most basic thing ever. Like when your mom says, 'You should have known better,' but you didn't.
'Had had? What's that?' 'That's what you get when you forget to study.'
'You had had the chance to do your homework!' 'I had had the chance to do my homework... and I ignored it.'
'Had had is the easiest thing in the world.' 'Then why did you fail the test?'