Similar to a steam steriliser or bottle steriliser, which use steam to sanitise breast pump parts and other accessories, Medela microwave bags use steam to rapidly and efficiently kill germs, common bacteria, and yeast. There is some preparation required, just like with any other sanitization procedure. Rinse the objects you need to sanitize with cold, clean water, then with warm, soapy water, and finally rinse again. After that, put your parts in the bag with a little water and shut it. Put in your microwave and cook for at least three minutes, as directed by the instruction booklet. After finishing, gently take the components and water out of the bag and pat dry with a fresh cloth. Refer to the product box for more instructions.