How do cats and dogs know which is cat food and which is dog food?

I have 2 cats and three dogs. One night, I ran out of cat food (the dry type) and my cats were hungry. So since I had dog food only and the shops were closed, I gave them dog kibbles. Theyre more or less of the same shape and size and are made of the same ingredients - chicken. My cats took a nibble and refused the rest of the bowl, preferring to go hungry than eat dog food. The next day, i got them a fresh pack of made-for-cats kibbles and they immediately dived in and ate up. How come my cats knew what I gave them wasnt cat food? I also know of dogs that wont eat cat food for the long run (not mine though, cos theyre pigs and will eat anything). It would be good if a pet food manufacturer can answer this question. Is there some special ingredient you put in cat and dog food that makes cats and dogs know which kibbles is whos? Also, nowadays, dogs are eating fish too - like salmon. You cant say its the ingredient. How come they know the difference?