Similar to a mortar and pestle, a muddler is a tool to mash certain ingredients like spices, herbs, and fruits for drinks. They are usually made of stainless steel or wood and are about eight to ten inches long to reach the bottom of a glass or cocktail shaker.  If you do not have a muddler, you can use a wooden spoon, a mortar, and pestle, a tamper, a meat mallet, or even a dowel. A French rolling pin would also work for muddling your mojito ingredients.