Solution:
As always, try to find a way of looking at the problem that makes the solution simple. In this problem, regardless of the current state of the puzzle, a move consists of combining two clusters into a single cluster. Therefore every move reduces the number of clusters by 1. Since we begin with 2000 clusters (the single pieces) and end with 1 cluster, we need
\[2000-1=\boxed{1999}\]
moves to complete the puzzle.
Let's say that you have two pieces of paper that both represent planes. If you cross them, you'll be able to see that the intersection of these two pieces of paper is a line.