Explaining Islam to one who is asking/ mocking:-
1.) Do not feel shy
2.) Speak the truth
3.) Turned it into an opportunity to explain Islam to him
Muslim narrated in his Saheeh (386) that Salmaan al-Faarisi (may Allah be pleased with him) said: The mushrikoon said to us [according to Ahmad and others: One of the mushrikeen said to him when they were making fun of him]: I think that your companion is teaching you everything; he is even teaching you how to defecate!
Salmaan said: Indeed, he has told us not to clean ourselves using the right hand and not to face towards the qiblah (when defecating), and he has told us not to use dried dung or bones, and he said: “Do not clean yourselves using less than three stones.”
See how this noble Sahaabi responded in this case having to do with the etiquette of relieving oneself and going to the toilet, without feeling any shame or embarrassment.
Assuming that the one who asked him this question was mocking him, he turned the tables on him, as they say, and turned it into an opportunity to explain Islam to him.
And Allah knows best.
