Water at 140° Fahrenheit can seriously burn an adult in just 5 seconds. It can burn a child even faster. Setting your water heater no higher than 120° can protect everyone in your home from burns, and you will still be able to take a hot shower.
Water heaters often do not show the correct water temperature. So to check the temperature for yourself, use a thermometer that shows high temperatures, like one for meat or candy. Turn on the hot water tap in your bathroom, and fill a cup with the hot water (so that you don’t burn yourself by trying to stick the thermometer under running water). If the temperature is higher than 120°, follow the water heater owner’s manual (available online) to turn the temperature down. That will mean calling in a professional in some cases. But to prevent a burn, it’s worth it.