Top: An episode from Exodus 15:22-25. The Israelites complain to Moses about the water (left); God shows Moses a tree to put in the water to make it sweet (right); the Israelites drink (center). Bottom: The Amelkites emerge from a walled city to do battle (Exodus 17:8). Detail of mosaic in the nave, set up by Pope Sixtus III (432-40). Basilica Papale di Santa Maria Maggiore, Rome, Italy.