Lemon juice can help lighten the skin on elbows and knees. Gently rub half a lemon onto your elbows and knees. Repeat this procedure over a period of time and skin should brighten.
Lemons contain plenty of vitamin C and citric acid that can help naturally brighten darkened areas of the skin. The vitamin C in lemons is a natural antioxidant that can increase collagen production and help reduce the appearance of dark spots. Make sure to apply sunscreen when you head outdoors, as the citric acid in lemons can make your skin more sensitive to the effects of the sun.