The key to knowing when a baby needs mittens is to understand how babies lose heat. Babies lose heat mainly through their heads, necks, and chests. They can also lose heat through their arms and legs if they are uncovered. So when it is cold outside, you should cover your baby's head, neck, and chest with a hat, scarf, and jacket. And if it is really cold outside, you should also cover your baby's arms and legs with pants and boots.If your baby is still losing heat even when they are all covered up, then you may need to put mittens on them. Mittens help keep your baby's hands warm by trapping the warm air close to their skin.There are a few things to keep in mind when buying mittens for your baby. Make sure the mittens are not too big or too small. The mittens should fit snugly around your baby's hands so that they cannot remove them easily. You also want to make sure that the mittens are made of a thick material so that they will keep your baby's hands warm.Now that you know how to tell when a baby needs mittens, you can keep your baby safe and warm during the cold winter months!