Utkatasana (chair pose)
Stand in Tadasana. Inhale and raise your arms above your head with the palms of your hands facing each other. If you notice that you are hunching your shoulders, relax them, but if you they're still up close to your ears, you can separate your arms further and bend your elbows, in an "Egyptian style".
Exhale and bend your knees, sitting back as if you were going to sit down on a chair. Make sure that your knees don't go past the tips of your toes. Sit as far as you can, tucking your tailbone in to lengthen your lower back, and holding the navel in towards the spine to keep your core muscles engaged.
Hold this pose as long as you can, breathing rhythmically. Come out on an inhalation and exhale when you lower your arms.
BENEFITS: strengthens your thigh muscles and is therefore highly recommended for people with weak knees (you strengthen the knees by working the muscles around them). It also strengthens the arms and stimulates the diaphragm.
CONTRAINDICATIONS: Although I've just said that this pose helps strengthen weak knees, if you have serious knee injuries you should consult a doctor first. Also, people with lower back pain may find this pose uncomfortable if they don't lengthen the lower back and tuck the tailbone in.