Soup in a skull shaped bowl.
When you inspect the soup you get the option to drink it. If you drink it, it will say "Viola got poisoned" and then you hear a unlocking sound. Since you are poisoned, every step you take makes you take damage until you you vomit blood and get a 'Game Over'.
When you optained the Silver Key you can hit the bowl with the key. It will make a pleasent sound. You can also put the key in the soup. The key will turn black in the soup since (as said in the book The Dining of Aristocrats) and you can continue with the game.
In Extra mode, there's a cover covering the soup, if you open it a slug looking thing will jump up on Viola's face and you will get a 'Game Over'. You simply can't open the lid in Extra mode, you have to give the Bloody Limbs to The Cook first. He will chase you, when you run out of the diner room he will stop chasing you and go back to the kitchen, while going back to the kitchen he will knock over the lid. That's when you can find and put the Silver Key in the soup. There will be two chairs around the Soup. One with the Silver Key and one with a Slug looking thing waiting to jump up in Viola's face making you get a 'Game Over'.
Apperently Ellen's Predecessor actually drank the green looking poisonus soup and liked it.