When I first introduced cow's milk in Leah's sippy, she refused it too. It took about 2 weeks of milk in her sippy at every meal time before she got the hang of it. Persistance is the key. Also, if your LO is used to getting water or juice out of a certain sippy cup, you may want to introduce a different cup for milk. A straw cup is always nice - otherwise a normal cup that you hold. Leah LOVES to drink out of normal cups. She needs lots of assistance, but she thinks she's pretty cool drinking from a cup like Momma!