Take a piece of thread and tie a small place of stone at one of its ends. Hold the other end of the thread and move the stone to describe a circular path with constant speed.

A stone describing a circular path with a velocity of constant magnitude
Now, release the thread.
Can you tell the direction in which the stone moves after it is released ?

Explanation

When the stone is released, it moves along a straight line tangential to the circular path. By repeating the activity for a few times and releasing the stone at different positions of the circular path, check whether the direction in which the stone moves remains the same or not.
Observation: When the stone is released, it moves along a straight line tangental to the circular path.