Do mammals bear live young?

No, there are two mammals that lay eggs - the platypus and the echidna. All others give live birth.
All mammals do in fact bear live young. This is one of the major characteristics of a mammal, which also includes being warm blooded, breathing air, and having milk for young (which is where the root "mam" comes from in mammal). Mammal also means "from the breast" which is where the milk is made.

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