My eldest daughter Alex is just 16 and quite a sweetie, recently she has really started to take an interest in herself and spending a small fortune on nice underwear. She has a good figure and 34dd up top which she is proud of. Tidying her room I recently came across a few lingerie catalogues and her panty drawer is full of little silky numbers. The other day I came back from the shop and thought she was still asleep so I went to wake her for work, her door was ajar so I peeked around and she was lying on top of her bed just in her underwear listening to her ipod, then I noticed she was looking at a lingerie catalog, she had her legs up and apart and she was rubbing herself over her panties with a huge fat marker pen, I froze on the spot. I had seen her masturbating a few times before and when she slept with me when she was younger I felt her fingering herself during the night, but seeing her on her bed in sexy underwear masturbating was very sexy and I felt my own pussy moisten. She felt up her tits and I could make out her nipples so stiff, as I watched her I began to tremble, then I noticed she had some more of her panties on the bed and she was rubbing them between her thighs as well as the marker pen, then her fingers would slip under the elastic and she'd pump away for a bit, I knew I should go but it was so horny, finally as her hips began to rise off the bed she tugged the front of her knickers up into her slit and rode them until she came, I dived out and went to the bathroom to bring myself off, what a moment!