One day I was cooking some yellow and green squash fresh out of my garden and couldn't wait to serve it for dinner. I decided I would add some portabella mushrooms that had been in my fridge for a few days. I cut off the stems and sliced them up adding them to the squash mixture. The one thing that I forgot to do was remove the "ribs!!!" Immediately everything turned this ugly grey color. We ended up having pizza for dinner that evening. Now, I always remove the ribs and only cook portabella's on the grill!!!!