Some gases are lighter than air. Hydrogen is the lightest of all, and easily manufactured, even at home -- for example by dropping aluminium foil into a solution of sodium hydroxide (highly dangerous because it will spit droplets of hot caustic liquid at you which may blind you permanently if they land in your eyes).
The corpus luteum is essential for establishing and maintaining pregnancy in females. The corpus luteum secretes progesterone, which is a steroid hormone responsible for the decidualization of the endometrium (its development) and maintenance, respectively. It also produces relaxin, a hormone responsible for softening of the pubic symphysis which helps in parturition.
Days 14-28 of the menstrual cycle: The ovaries are in the luteal phase and the uterus is in the secretory phase in the ovarian luteal phase. Luteinizing hormone (LH) induces the formation of a corpus luteum (which degenerates if pregnancy does not occur). In the uterine secretory phase under the influence of estrogen and progesterone, the endometrium continues to grow. The glands become enlarged and if implantation does not occur the endometrium is shed after 28 days. Generally by day 28 of the cycle the endometrium measures between 10-16mm.