What are quarks made out of?

Question: What are quarks made out of?
- Keywords:
  quark,
  subatomic particle
Asked by Zealousy to Rob on 16 Jun 2015.
- Rob Temperton answered on 16 Jun 2015:
  
  As far as we have any experimental evidence for, quarks are elementary particles – ie they are the fundamental building block. However, this could well be because we have not managed to create an experiment that has enough energy to rip them apart.
  
  Some theorists have come up with many wonderful/crazy ideas – the most famous of which is string theory. I am not an expert (I have never studied it), but the basic idea is that all the particles are made up of a ‘string’. The ‘frequency’ at which the string vibrates determines what kind of particle it is. This is still very much an idea and there is no experimental evidence supporting it that I am aware of.
  
  Rob
  
  PS – if you are going to ask me what a ‘string’ physically is, I have no idea!!!

Question: What are quarks made out of?