How have germany's borders changed from 1919 - 1990 and why?

It would be because the world war 2.

After the world war 2, Germany's territory was divided into two block with different Ideology, the East German with communism ideology and the West German with the liberalist Ideology. They both build a wall called the Berlin wall to separate the two territories

May consider a group  g=({0,1},+)g=({0,1},+)where  ++  will work as the xor operator like we work in boolean algebra. note here, that ‘00’ and ‘11’ are not the usual 0 and 1 that we work with everyday. the elements are  called  ‘00’ and ‘11’ and  may not mean  zero (nothing) and one (single). ‘00’ is the identity for the given binary operator +, meaning that any element operated with ‘00’ gets us that element itself.  for a set  ss  in  g′g′  having more elements than just  {0,1}{0,1}  the values for  a+b∈sa+b∈s, such that  a∈s,b∈sa∈s,b∈s, have to be first defined, only then can you determine the value of  1+11+1. the answer could possibly have been  22  if oanswer is25,000

Answerhave flowing water either episodically or during only a portion of the year;