sentence2_structure: Incubation<>is<>when birds keep their eggs warm while the embryos inside develop$$$birds<>keep<>their eggs<>while the embryos inside develop$$$the embryos<>inside develop<>
sentence1: Incubation is the process of keeping the egg warm so the baby is protected while it's developing.
sentence2: Incubation is when birds keep their eggs warm while the embryos inside develop.
question: What is it called when birds keep their eggs warm while the embryos inside develop?