&lt;div&gt;&lt;span style=&quot;font-size: 20px;&quot;&gt;&lt;font color=&quot;#000000&quot;&gt;&lt;b&gt;Obedient Ball&lt;/b&gt;&lt;/font&gt;&lt;/span&gt;&lt;/div&gt;&lt;div&gt;&lt;span style=&quot;font-size: 20px;&quot;&gt;&lt;font color=&quot;#000000&quot;&gt;&lt;br&gt;&lt;/font&gt;&lt;/span&gt;&lt;/div&gt;&lt;div&gt;&lt;span style=&quot;font-size: 20px;&quot;&gt;&lt;font color=&quot;#000000&quot;&gt;The magician displays a ball threaded on a string. The ball slides freely up and down on the string. Yet whenever the magician wishes he causes the ball to freeze on the string. The audience may be invited to shout 'STOP' or 'GO', and whenever they shout &quot;STOP&quot; the ball freezes on the string and when they shout GO it slides down. &lt;/font&gt;&lt;/span&gt;&lt;/div&gt;&lt;div&gt;&lt;span style=&quot;font-size: 20px;&quot;&gt;&lt;font color=&quot;#000000&quot;&gt;Easy to do.&lt;/font&gt;&lt;/span&gt;&lt;/div&gt;