Why is it that when you stare at something for too long, two images merge together, so you see two different things?

Question: Why is it that when you stare at something for too long, two images merge together, so you see two different things?
- Keywords:
  eye,
  image,
  merge,
  perception,
  stare,
  trick,
  vision,
  visual
Asked by mennie13 to Helen, Jenni, Mark, Martin, Stu on 20 Jun 2011.
- Jenni Tilley answered on 20 Jun 2011:
  
  Do you mean like ‘Magic Eye’ pictures? It’s because both our eyes seem a slightly different image. When you relax your eyes’ the two image merge into one.

Question: Why is it that when you stare at something for too long, two images merge together, so you see two different things?