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You say 'haha, that's funny' when you're pretending to laugh but you're really just annoyed and think the joke is the worst thing ever.
'Haha, that's funny' when my friend told me a joke about a sad potato.
'Haha, that's funny' when my teacher made me write 'I will not talk during class' 50 times.
'Haha, that's funny' when my dog tried to eat my homework.