Undo or Backup function
This relates to the Android app (and maybe other platforms.) One of the exercises involves creating a sentence by clicking on a series of word buttons. If you inadvertently click on the wrong button there's no way to back out of it or "undo" your error. The only options seem to be either to continue in the knowledge that your answer is wrong or quit the current lesson and start it all over from the beginning. These options seem unually harsh considering that in other exercises you can use the keyboard backspace key to back up or undo an error. It's also negative reinforcement in that even though you can clearly see you're making an error you can't stop it. Maybe I'm missing a way to do this. Dragging the button doesn't work. So if there's a way to do this already please tell me. If not, isn't this something that needs to be in the user interface?
You should be able to click the misplaced word in order to remove it from the sentence you are building. Is this functionality not working for you?