Barometer
 You can make a simple homemade barometer by attaching a cut off balloon to the top of a glass bowl, then attaching a straw to that. The results are a makeshift tool that in inflates with air pressure.

As the straw goes up we know that there is a high pressure pressing down on the balloon, making the straw go up.

As the straw goes down we know that there is a low pressure letting the balloon inflate, making the straw go down.

In other words, as we present in this graph, the height of the straw and the air pressure is positive correlated.