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Combine equal parts of these ingredients by weight in a bowl. You can get plaster of Paris at craft stores or home improvement stores and cornmeal at any grocery store.  If you don't have a way to weigh it, try 2/3 cup (110 g) of each. If you don't have cornmeal, try using flour instead in the same proportions.  The plaster of Paris will get harder in the rats' stomach, eventually killing them. This step is optional, but the sweetness of the sugar will encourage the rats to eat more of the mixture. After adding equal parts of plaster and cornmeal, add half a part of sugar. Pour the milk into the powdered mixture. You may need more milk as you go along, but start with this much so you don't make the mixture too wet. If you don't have milk on hand, you can just use water. The milk will give it more flavor to draw in the rats, but most likely, they'll still eat it just for the cornmeal or flour. This mixture isn't toxic to humans, so it's not a problem to use your bare hands. However, if you don't want to get your hands all sticky, you can put gloves on.  If the mixture isn't sticking together and you see loose powder still, add more water or milk, a spoonful at a time. You want it to form a dough you can roll into balls like clay. If it seems too liquidy, add more plaster and cornmeal/flour in equal parts, only adding a spoonful at a time until you get the right consistency. Pinch off some of the dough and roll it between your hands to form a small ball. You can make them even smaller if you prefer. The rats will eat them either way. Place the balls where you see evidence of rats (out of reach of children and pets), and check back in a day or 2 to make sure they're eating the balls. If they're not, you may need to move the balls. If the rats still aren't interested, you may need to make a new set.
Mix 1⁄4 lb (110 g) of plaster of Paris and 1⁄4 lb (110 g) cornmeal in a large bowl. Add 1/3 cup (55 g) of sugar to make it more enticing. Start with 1 cup (240 mL) of milk. Knead the mixture together with your hands. Roll the mixture into balls about the size of golf balls.