A method of transmitting an inquiry message, performed by a display device, the method comprising: reproducing content in the display device; receiving (S220, S610) an input from a user; determining whether the input from the user is related to the content, and only if the input from the user is related to the content, generating an inquiry message about the content based on the input from the user; obtaining timing information of the inquiry message by analyzing the user input, the timing information indicating at least one of: a time at which reception of the user input began, and a time at which reception of the user input ended; extracting information about a frame of the content that is reproduced when the input from the user is received, by selecting the frame based on the timing information, and extracting the information based on the input from the user; transmitting the extracted information and the inquiry message to an external device that is connected with the display device wirelessly or by wire; and receiving input information of a user of the external device, from the external device, and displaying the inquiry message and the input information of the user of the external device on a share window for sharing information with the external device, wherein the share window includes one or more of a shared memo-pad, a shared white board, and a message window; wherein the determining comprises: extracting at least one meaning unit from the input by parsing the input from the user; and determining whether the input from the user is related to the content by determining a relation between the at least one meaning unit and the content, based on metadata of the content. reproducing content in the display device; receiving (S220, S610) an input from a user; determining whether the input from the user is related to the content, and only if the input from the user is related to the content, generating an inquiry message about the content based on the input from the user; obtaining timing information of the inquiry message by analyzing the user input, the timing information indicating at least one of: a time at which reception of the user input began, and a time at which reception of the user input ended; a time at which reception of the user input began, and a time at which reception of the user input ended; extracting information about a frame of the content that is reproduced when the input from the user is received, by selecting the frame based on the timing information, and extracting the information based on the input from the user; transmitting the extracted information and the inquiry message to an external device that is connected with the display device wirelessly or by wire; and receiving input information of a user of the external device, from the external device, and displaying the inquiry message and the input information of the user of the external device on a share window for sharing information with the external device, wherein the share window includes one or more of a shared memo-pad, a shared white board, and a message window; wherein the determining comprises: extracting at least one meaning unit from the input by parsing the input from the user; and determining whether the input from the user is related to the content by determining a relation between the at least one meaning unit and the content, based on metadata of the content. extracting at least one meaning unit from the input by parsing the input from the user; and determining whether the input from the user is related to the content by determining a relation between the at least one meaning unit and the content, based on metadata of the content.