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How to fake a headache
Know your symptoms.
You can't credibly fake an illness if you don't know what is wrong with you. If you are complaining about a terrible headache, make sure you know exactly the kind of headache you want to fake so you can explain to others what is wrong with you when they ask.
If you are trying to get out of an event, make sure you emphasize how bad your symptoms are. This way, it won't seem as if it is not worth letting you skip whatever you are trying to avoid.