Because she wants to give you some underwear 🤣 Maybe she's got a bag of hand me downs and trying to suss if you'd like it. She can't possibly know your MIL buys you loads of stuff, unless you've specifically told her that
People don't think of underwear for children in the same way that they think of underwear for adult. Well, not normal people anyway. Kids underwear has lots of cartoon characters on them, and sometimes that actually encourages kids to learn to hold because they don't want to pee on them. And like Ella said, potty training is very time-consuming and can be expensive with clothes etc. It's actually quite thoughtful.
I don’t think it’s that serious lol.
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Probably adoptive parents encouraged the family to make more of an effort . Seems random and weird to you . But from another prospective it's actually thoughtful. Potty training is expensive and challenging. Usually encouraged to start 1.5years/2,years old. How old is your baby