Many animals go on long and difficult journeys to find food or places for their young to start life. They often travel very far, sometimes even halfway around the world. Some animals make one of these amazing journeys every year, while others make only one amazing journey in their lives. Some animals travel over land, some through water and others in the air. Green turtles spend most of their lives in the sea, but they come to the shore to lay their eggs. Every two or three years, fully-grown female turtles swim a long way to the same beach where they were hatched   and lay their eggs there. Some turtles may travel over 1240 miles to reach the same beach. No one knows how these turtles are able to find the same beach again. Monarch butterflies live in North America. At the end of summer, they fly south to Mexico. In spring, they fly back north. During this journey, the adults lay their eggs and then they die. When the young butterflies hatch, they continue the journey north. No one knows how the young butterflies know where to go. The main idea of this passage is   _  .
about some animals' looks about some animals' journeys to find food or lay eggs why some animals lay eggs when some animals find food