Here is a story:
A chap called Billy spent a night on a sofa. In the morning Billy helped himself to a breakfast of whisky and cat-food. He did declare "''tis the finest gravy you ever did taste." (or very similar)
This story had been around a bit, and exploited for amusement. I thought I could get some more amusement from it...

Eating cat-food "freaks people out", it's a potential party-trick and April-fools' joke, so here's how you can do it.

Step 1: Things You Need

Two aluminium tins of cat-food, of the type pictured. Choose whatever flavour you like, but aim to have the filling match the description.
Red-meat and offal, I used steak and kidney.
Gravy, I was lazy and bought a tub from a butcher.
A sharp knife, scissors.
Epoxy glue.
A kitchen, with kitchen-stuff in it.

Step 2: The Meat

I got a little portion of steak and kidney, about half a pound.
To make it look like horrible processed brown meatyness it needed to be pulped to mush and cooked. I chopped steak and kidney finely and tried bashing it with a pestle, but used a hand blender in the end.
*Use a blender*
For flavour, I added a little salt and pepper, and half a teaspoon of Marmite.
If you're recreating two-tone cat-food, make two mushes with different proportions & flavourings.

The mush was poached gently in a lightly-oiled ramekin to a rather nasty-looking brown lump. This was easily cut into horrible-looking chunks with a sharp knife. The chunks taste nice though - remember this: they look nasty but taste nice.

To finish off the lumps were re-heated in rich brown gravy, yum yum...

Step 3: The Tin

To be convincing it's essential that you open what looks like a tin of cat-food in front of people. Better still to get someone else to open it for you, this is half the cleverness of the joke.

Get two tins, of the type you see pictured.
Pull the lid from one, empty it and thoroughly clean it.
Snip the curled ridge off neatly with scissors, take time and make it look tidy.
Push this empty tin onto the other tin, such that it fits snugly. The snipped-edge needs to sit neatly in the other tin, nice and "flush", so re-trim it where necessary.

Then carefully cut a hole in the bottom of the other tin, empty it and thoroughly clean it.
Now you have two halves, the join will only be visible once the tin has been opened and you get to the bottom of it (the joke will be done by then though)

(My cat-food was "donated" to local wildlife)

Step 4: Filling and Finishing

The unpleasant-looking lumps were heated up in gravy, then spooned out into the tin with a hole in the bottom. Then the tin was topped up with gravy - Do not over-fill this, you'll have problems gluing it together if you do.
I applied rapid 2-part epoxy to the edge of the outer-tin, as far away from the brown meaty stuff as possible. Once the two were firmly pressed together the glue set rather quickly, helped by the heat from the hot meat and gravy.
Ensure to wipe any excess glue off while it's still fluid.

Step 5: Pulling the Joke

I was already set-up with the Billy story. People had talked about how fine the gravy was, so it easy to say "let's find out how good this gravy really is". Note from Kiteman: On a side note, hygiene standards in pet food manufacture are allegedly higher than for human foodstuffs, so, taste aside, tinned petfood is safer to eat than "proper" tinned foods. Use this if it helps.
I packed cocktail-sticks and some bread to mop-up the lovely gravy - it worked, several people refused to taste it, at least one was visibly disgusted _{(ha ha ha ha)}
You might want to get a friend to back you up and encourage others to join in, two people eating is more persuasive than just one.

When the tin is empty the "tampering" will be visible, so either show people or dispose of it before you get to the bottom.