How is one way hashing possible?

How is one way hashing possible?

I was wondering, how is one way hashing possible? how can you encode a
string in only one direction and not the other? If you reverse the
algorithm, can't you decode something like md5? Before, to me, the only
way that is possible is to have it generate a random answer instead of
using an algorithm. But it hashes the same way every time. Please explain
this to me.