"Endosymbiosis is like having a one night stand turn into a life-partner. You, a prokaryote, invite a mitochondrion in, with hopes to eat it later. The mitochondrion decides it doesn't want to be eaten, and sticks around to enjoy your nutrient-rich house while you get your kicks from the energy the mitochondrion brings about. Then, you asexually multiply, and all your kids have mitochondrion inside them too. This fits in no way with the metaphor, but it's what happens."