If it walks out of the refrigerator, leave it alone.
Milk is spoiled when it starts to look like yogurt.
Yogurt is spoiled when it starts to look like cottage cheese. Cottage cheese is spoiled when it starts to look like regular cheese. If opening the refrigerator door causes stray animals from a three-block radius to congregate outside your house, toss the meat. Bibb lettuce is spoiled when you can't get it off the bottom of the vegetable crisper without Comet and a brillo pad. A carrot you can tie into a clove hitch is no longer fresh.