人が時間をさかのぼって、祖父母が何らかの形で子供を産むことを妨げた場合、祖父母が子供を産んだことがないので、時間をさかのぼって時間をさかのぼって親がいないでしょう。つまり、両親は存在しないため、その人は存在できませんでした。それで、これは質問を頼みます。その人が存在しなかった場合、誰が時間をさかのぼったのですか？それがパラドックスと呼ばれる理由です。
If a person went back in time and prevented their grandparents from having children in some way or another, the person who went back in time wouldn't have parents since their grandparents never had children. Which means the person couldn't exist since their parents don't exist. So this begs the question, who went back in time if that person never existed? That is why it's called a paradox.