Sanji's flames in One Piece are something that many fans wonder about. Is that flame a product of technology or where does it come from? Why isn't Sanji burned? This article will reveal the author of One Piece's thoughts on this issue.
Sanji in One Piece talks about the origin of fire
In the manga, when Sanji was asked how he was able to burn his legs, he stated that it was his burning passion that allowed him to do so and that was also the reason why he himself was not burned. Simply put, because his heart was full of fire and passion, he was not harmed by his burning legs.
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However, quite a few fans immediately knew that this was not the real reason, simply because Oda is very good at creating this kind of smokescreen. The first point that readers realized was Sanji's fight in One Piece with Queen. Queen made it quite clear that no human can spontaneously burst into flames, this is not a human ability, no matter how much passion Sanji's spirit possesses. Although Sanji later affirmed that the fire in his heart burned hotter than any other fire, readers gradually doubted the origin of this fire.

Information revealed by author Eiichiro Oda
Oda finally revealed the origin of Sanji's flames in One Piece episode 105 SBS.
“D: I have a question. In Chapter 1031 of Volume 102, Sanji's eyebrows were inverted, but in Chapter 1044 of Volume 103, they returned to normal. Why did they return to normal?
O: You noticed it~. His eyebrows have changed. Back in the Whole Cake Island Arc, all of Sanji's siblings had superhuman qualities and healthy skin called "Exoskeletons" because they were born due to the "Lineage Factor". However, only Sanji was born as a normal human due to his mother's resistance. Look at his siblings' eyebrows, everyone has "66", like Yonji. Sanji is the complete opposite. It was a battle where the "power of science" was shown in the Raid Suit! Sanji already had an abnormal resistance to fire. How strange! Please continue to watch.”
The most interesting part here comes at the end, when Oda mentions that Sanji in One Piece has an unusual resistance to fire. Fans have seen this throughout the series, right from the Baratie arc and in the Enies Lobby arc, this resistance to fire was taken to the next level when he became Diable Jambe. In Wano, this ability became even more terrifying in the form of Ifrit Jambe. According to Oda, this resistance to fire is very strange and he directly links it to Sanji's ability to manipulate his bloodline and the scientific advances that Judge has made and used on his body.

According to Oda, fans should keep a close eye on Sanji's power in One Piece as it could very well be an important plot point. Although readers now know for sure that Sanji's flames are a product of scientific power and could very well be related to other Devil Fruits. For more details, we'll have to keep reading One Piece!