Some recipes may add a number of ingredients, such as stock and seasonings (for taste) or seltzer or baking powder (for fluffiness). Traditionally, the fat had been schmaltz (chicken fat), which imparts a distinctive flavor, but vegetable oils or margarine are usually used by the more modern, healthwise chef. Butter is not used as milk products are not allowed to be used in chicken (meat) soup in accordance with the rules of kashrut. There are even recipes for fat-free Matzah balls.
The balls are shaped by hand and dropped into a pot of salted, boiling water or chicken soup. Keeping one's hands wet is vital when handling the sticky dough. The balls swell during the boiling time of approximately 20 minutes, and come out light or dense, depending on the precise recipe. Matzah balls are roughly spherical and can range anywhere from a couple of centimeters in diameter to the size of a large orange, depending on preference. They can be frozen and reheated in soup.
Matzah balls are usually served with chicken broth as matzah ball soup.
Another variation is called Matzah Kleis. They are made as follows: Break a whole matzah into little pieces, and soak it in water. After half an hour, drain off and squeeze out the surplus water. Add one to two dessert spoons of matzah-meal, salt and pepper to season, and then some fried onions, chopped (before frying) into very small pieces. Then add a tablespoon of olive oil and a whole egg. Knead well, until completely mixed, and put the mixture in the fridge for half an hour. Heat the chicken soup to almost boiling. Roll the matzah mixture into small balls (about an inch in diameter, as they will swell up) and drop them into the chicken soup, one by one. You can also add chopped parsley, if you like.
Matzah balls are particularly popular during Passover, when matzah meal is often used in observant Ashkenazi Jewish households as flour may not be used. (Those Ashkenazi Jews with the custom against gebrochts, however, would not eat them on Passover -- see the gebrochts article for more.) They are also eaten at other times of year, especially on Shabbat, as a quintessential comfort food.