what is difference between drinking from a teat vs spout vs open cup

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my toddler exchnages between all and seems ot drink in the same manner

yet folk write a lot about weaning off teat

why?

teat is just nice for him when with milk

but everything else he knows is in a cup etc

Sucking on a teat can affect the position of his teeth and bring the drink into prolonged contact with them causing erosion (even milk contains sugar).

Mess!

We have open cups in the kitchen and lidded cups in the living room.