How Do They Know?

I had a phone call telling me that T*sco were selling off some dry cat food. As the owner of a couple of naughty moggies, I took up the kind offer to pick up some of bargain NutriCat.

It was nearing it's sell-by date, but as I don't believe in these dates, I said, 'OK, get me 10 bags'.
They were 2.5kg bags - er.... that's 25 kilos of the stuff. Great, that will keep me going for a very long time.

As you can see, they were reduced from £8 a bag to 40p. So, £4 for 25kg of dry cat food - likely to last for ages.

Happy pussies all round and loads of money saved? Well, no.
I took all the usual precautions - didn't bring it into the house all in one go, never mentioned I had mountains of the stuff, removed the yellow stickers, so they wouldn't know it was reduced, etc.
First 3 days were fine - proved to be very popular at meal times.
But........
Will they eat it now? - will they buggery.
How do they know?
Think the remainder is destined for the local animal sanctuary!