Should spoken-language interfaces incorporate human discourse phenomena? Acknowledgments, for example, are ubiquitous in human conversation but are rare in human-computer interaction. Are people unwilling to use this human convention when talking to a machine, or is their scarcity due to the design of current spoken-language interfaces? We found that, given a simple spoken-language interface that responded to acknowledgments, over two thirds of subjects used acknowledgments at least once, about the same number that used more traditional commands to control the interface. These results were consistent for both Mexican Spanish and American English versions of the interface, and they suggest that it may be possible to make use of human discourse mechanisms such as acknowledgment to build more flexible spoken-language interfaces.