Even though they have experienced a surge in popularity in recent years, kettlebells have been used for many years in strength training and metabolic conditioning because the movements are easy to learn. One of their best traits is that they are compact and easy to store, making them an excellent exercise option if you have only a limited amount of space and time for exercise. For best results, invest in two kettlebells: one relatively light, the other heavy enough to cause fatigue by the end of the allotted time frame for each circuit. For most people, the heavy kettlebells are 10 to 12 kilograms (22 to 26 lbs).