A man goes to the doctor and says: 'Doctor, it really hurts when I touch my knee like this.'
The doctor replies: 'So don't touch your knee like that.'
Stupid? Perhaps. But this joke, like many of the other jokes you'll see in this forum, illustrates a very important use of the Present Simple tense in English. Did you know that an estimated 70% of what native English speakers say is in the Present Simple tense? 70%!!
And yet, how much time do you spend at school and in formal English classes learning about the Present Simple tense? Answer: very little.
One of the most common uses of the Present Simple (which is rarely covered in mainstream grammar books) is the 'Story-telling' use. Native speakers use the Present Simple very frequently to tell stories and jokes. The joke above should really have been told in the Past Simple tense, because it happened in the past, but any native speaker of English would tell you that it sounds much more natural when told in the Present Simple.
Try to tell your next joke in English using the Present Simple. Just remember that jokes are normally told verbally, not in writing, so they might not seem quite as natural when read on paper/online in this way. Not to worry - the practice will do you good and will help you master the essential art of telling 'natural' stories in English.