From promising Abraham and Sarah that they'd have a baby when they were both over 90 years old, to telling the people of Israel to march around in circles to defeat the city of Jericho, to having David face a giant with just a sling and a stone; there are times in life when God asks us to do impossible things. But God has a habit of making the impossible possible.
God does this to show how powerful God is and to show us that we should always trust in him.
