Samadhi can be compared with an island in the sea on which a swallow has built its nest. If we are to catch the bird, we need not follow it anywhere. We simply wait on the island. Although it may fly away until out of our sight, it will not be able to find another island. It can see nothing but the sky and water and so, in order to survive, all it can do is to return to the island. We can catch it right there.
Our mind has a similar nature. It does not matter how far our thoughts wander. In the end they will start again in the same place. There may be all sorts of thoughts: about the past, the future, making a living, getting rich, and they go on continuously, but finally they will all begin again at the same point which is our mind. We must wait there as if on that island. After a while, the thoughts, like the swallow, become too exhausted and are forced to return to their nest, the mind.
If we always concentrate and are aware of what happens or where our thoughts are, in no time at all, the flight of thoughts will stop and then begin again within the mind. By keeping a close watch on that spot we will be able to catch it.