Craving more kefir and only have a bit left? Just add 1 part store bought kefir or homemade and your 3 parts milk  (optimally it is raw or pastuerized, not ultra-pasteurlized), and ferment as usual, I think a 1:3 ratio is optimal and speeds the process along, a glass jar with a coffee filter and rubber band out on the counter until it has thickened. If it separates, you can stir it back together. Avoid metal when using kefir. No need to strain! Just save a cup for your next batch ;)