The menstrual cycle is from Day 1 of bleeding to Day 1 of the next time of bleeding. Although the average cycle is 28 days, it is normal to have a cycle that is shorter or longer. Girls usually start having menstrual periods between the ages of 11 and 14.
In nuclear physics, nuclear fusion is a nuclear reaction in which two or more atomic nuclei come very close and then collide at a very high speed and join to form a new nucleus. During this process, matter is not conserved because some of the matter of the fusing nuclei is converted to photons (energy).
A huge amount of mass is above you, squeezing down on you from all sides. This is what happens to the hydrogen gas in the core of the Sun. It gets squeeze together so tightly that four hydrogen nuclei combine to form one helium atom. This is called nuclear fusion. In the process some of the mass of the hydrogen atoms is converted into energy in the form of light. The same process occurs in thermonuclear (fusion) bombs. In the Sun the process occurs in a controled manner. In a bomb it happens all at once in a big chain-reaction explosion.