The Crest Whitening Emulsions Pen is a leave-on teeth whitening kit that contains hydrogen peroxide as its active ingredient. With a size of 0.35oz, this product from the Crest 3D White product line is designed to provide a minty flavor and whiten teeth effectively. The emulsion formula adheres to teeth and provides a gradual whitening effect over time, making it a convenient and effective option for achieving a brighter smile.