Tutors make a list of idioms and transcribe them using QR codes. Tutees work with a device (teacher's cell phone) and decode the messages adding the idiom next to the QR code. 

(see Tutees at work at the bottom of the current page)

 black and blue

  catch red handed    

 the green light                                 

 ​​once in a blue moon

out of the blue

​cat nap

raining cats and dogs    

​​​black out

black sheep

​get the lion's share

see red

feel blue