40. A method comprising: under control of one or more computing devices configured to implement a virtual assistant, parsing user input with a natural language processing system that employs a language model, the user input comprising at least one of gesture input, audio input, keypad input, or touch input; representing the user input in a conversation user interface; determining, with the natural language processing system, a response to the user input; representing the response to the user input in the conversation user interface; enabling a user to interact with the conversation user interface to ascertain how the response was determined and to modify assumptions used to determine the response, wherein modification of one or more assumptions results in a modified response, and wherein individual assumptions comprise at least one of a value obtained via the conversation user interface, a value that is external to the conversation user interface, or a value that is obtained from a previous conversation between the user and the virtual assistant; and representing the modified response in the conversation user interface.
40. A method comprising: under control of one or more computing devices configured to implement a virtual assistant, parsing user input with a natural language processing system that employs a language model, the user input comprising at least one of gesture input, audio input, keypad input, or touch input; representing the user input in a conversation user interface; determining, with the natural language processing system, a response to the user input; representing the response to the user input in the conversation user interface; enabling a user to interact with the conversation user interface to ascertain how the response was determined and to modify assumptions used to determine the response, wherein modification of one or more assumptions results in a modified response, and wherein individual assumptions comprise at least one of a value obtained via the conversation user interface, a value that is external to the conversation user interface, or a value that is obtained from a previous conversation between the user and the virtual assistant; and representing the modified response in the conversation user interface. 43. A method of claim 40 , further comprising learning characteristics of the user and choosing the assumptions used to determine the response from the characteristics.