Dumbbell lunges:
Stand up straight with a dumbbell in each hand. Hang your arms at your sides. Palms should face the thighs (hammer grip). Feet should be a little less than shoulder-width apart. Take a big step forward with either leg, bending at the knee until the front thigh approaches parallel to the ground, landing on the heel. Inhale as you go down. The rear leg is bent at the knee and balanced on the toes. For the leg you step forward with, don't let the knee go past the tip of the toes. Step back to your standing starting position while exhaling. Repeat the desired number of repetitions.
The knee should never go beyond the sock. Sometimes it can, but just a little. Do not forget to work with your abs!

Dumbbell lunges.