Quickly Check for Understanding Using Plickers
"Plickers is an app that scans cards held by students to check for understanding. The teacher posts a multiple-choice or T/F question on the board/screen and the students hold up the card to show their answer. The Plickers app on your mobile device scans the cards and collects the data. The website shows the results in real time and allows the teacher to quickly assess the understanding or opinions of the class. Go to www.plickers.com to set up an account and go here to get the app."
(Thanks @Loater)
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