Million dollar question

We were working on spelling this afternoon and got down to the word "prettier" It confused her, so we spent some time working on 'y'-becomes-'i'-add-'er'. By the time we worked through that rule, she was fed up with spelling. Her next word was "should"

I read it to her in a sentence, and she looked at me, sighed and said "How do you spell should!?" Then she realized that it was a spelling exercise and started laughing hysterically.