Usability is everything – especially when it comes to mobile.

But creating a design that is truly user friendly requires an abundance of testing. This is where Plunk comes in handy. Plunk is a free Web app from ZURB that enables users to create click tests for design elements of a mobile app or website.

These tests reveal a lot about the usability of your mobile design, because they show where an audience clicks in response to a direct request. For instance, one Plunk example from Foursquare asks its audience to check into their current location. Once an audience member starts the test, they are presented with Foursquare’s app and have the opportunity to quickly click the requested call-to-action button.

In order to start testing with Plunk, users must visit plunkapp.com and upload a screen of a mobile Web page or application. Then, the test administrator can drag and drop a hot spot over the target that will be tested. After setup is complete, administrators can share the test, which is available for 48 hours, via email and social media in order to gather results. Then, test administrators can visit their reports page to view how many testers clicked the right location, which can help them decide if their site or mobile app’s design needs to be modified.