When babies spit up they usually do so because they drank too much formula or breast milk.
Babies mostly spit up soon or right after feeding and it's usually an indicator that the babies stomach is full and they no more room for anymore milk and so they spit up the excess.
Spit up is basically a nicer way of saying vomit and that's really what the baby is doing is vomiting that extra milk up.
The baby may also have gas built up in their stomach and that can also be another reason the baby is spitting up.
Babies will also sometimes spit up on you after they are burped so if you burp your baby you should have a washcloth on your shoulder.
It's happened to me as my kids have spit up on me when I was burping them so have a washcloth just in case.
Burping a baby is where you pick the baby up in your arms and then place them against your chest with their head facing the back of your body and then you pat the baby on the back.
As you pat the baby on the back gently it should cause the baby to burp out gas or air they had built up in their stomach while feeding.
Bottle fed babies usually get the most gas but even breastfed babies can also get air trapped in their stomach as well.
Burping the baby gets that gas or air out of their stomach so that they are not choking on it or become fussy.
After every feeding you should burp the baby or try to burp the baby to make sure the baby has no air trapped in their stomachs.
Otherwise they become fussy and cry and cry until you do relieve that gas in their stomachs by burping them.
Sometimes they may spit up on your shoulder while burping them so have a burp rag or washcloth on your shoulder before you burp the baby.