You can't shake Dr. Browns like you shake others. In order to mix up formula, I swirl them. If you shake, they leak. Also, as others suggest, transporting them without the disc may cause them to leak. I put mine in the side/end pocket of the diaper bag and sit it upright in the vehicle, at home, etc. I prefer that, to messing with the discs. I used Avent bottles when my older kids were babies and I liked them. This was 8+ years ago, so we weren't dealing with BPA free. I've heard the Avent aren't as good as they were many years ago. I like the Dr. Brown's ok, but if I had to do it again, I would have maybe tried a couple different brands, first. I had no major issues with colic, gas, etc. so maybe it was worth it. I'm also not sure that all had to do with the bottles. My sister was the one who raved about them, but also used the ones before the BPA redesign. I just didn't like all the parts and pieces to wash. But, back to your original question...they can't be shook to mix up formula.