Summarize this article in one sentence.
Treadmills are available at most gyms, and they're a convenient way to get some cardio exercise in and burn calories. Many treadmills let you adjust the speed and incline that you want to work out with, so it's a good choice regardless of your fitness level.  You can burn around 125 calories per 1 mile (1.6 km) walking briskly on a treadmill if you weigh 150 pounds (68 kg).  Using a treadmill can be hard on the knees, so you may want to use a different cardio machine if you suffer from knee pain or have a knee injury. Working out on an elliptical is also a good calorie-burning cardio exercise, and like a treadmill, you can usually adjust your intensity level. Ellipticals are popular exercise machines, so you shouldn't have a hard time finding one at your gym. You can burn 170-320 calories in 30 minutes on an elliptical, depending on your weight and the level of intensity you're working out at. For example, if you weigh 200 pounds (91 kg), you could burn 286 calories by working out on an elliptical at a high intensity for 30 minutes. Pedaling on a stationary bike at moderate intensity for 30 minutes can help you burn 200-700 calories, depending on your weight. It's also a great choice if you have knee problems since most of your body weight is being supported by the machine.  For example, if you weigh 150 pounds (68 kg), you could burn 250 calories by exercising on a stationary bike with moderate intensity for 30 minutes. If you're new to exercising, you can start out pedaling on a light intensity setting and then gradually increase your intensity level as you improve. Rowing is a great, full-body cardio exercise that won't place a lot of stress on your joints. On a rowing machine, you can burn calories while working out your arms, legs, and core. Start out on a low-intensity setting and progress slowly so you don't hurt your back. Depending on your weight, you can burn around 400-700 calories by spending 1 hour on a rowing machine. For example, if you weigh 240 pounds (110 kg), you could burn 650 calories by working out on a rowing machine with moderate intensity for 1 hour.

Summary:
Walk or run on a treadmill. Try using an elliptical machine. Exercise on a stationary bike. Work out on a rowing machine.