דריש ר"ע איש ואשה זכו שכינה ביניהן לא זכו אש אוכלתן
§ Rabbi Akiva taught: If a man [ish] and woman [isha] merit reward through a faithful marriage, the Divine Presence rests between them. The words ish and isha are almost identical; the difference between them is the middle letter yod in ish, and the final letter heh in isha. These two letters can be joined to form the name of God spelled yod, heh. But if due to licentiousness they do not merit reward, the Divine Presence departs, leaving in each word only the letters alef and shin, which spell esh, fire. Therefore, fire consumes them.
גחלת כמה דשביק לה מעמיא עמיא ואזלא
By contrast, with regard to an ember, as long as he leaves it alone it progressively dims, and the only way in which it will cause damage is if someone else actively fans it into a flame; therefore, he is not liable.
ואתא ישראל וחתי חתוי בגחלים - ...ומה שנהגו להשליך קיסם לתנור...