Walter's mother, Louise, has lost her home in a flood. Walter wants to make sure that his mother has somewhere to live for her remaining years and wants to allow her to have an ownership interest. Walter owns a plot of land near where his mother's home used to be. Walter wants to offer the land to his mother to own. However, Walter also wants to retain an ownership interest, so that he can build his own retirement home there one day. Explain what type of ownership interest Walter should establish to meet his goals of providing the land to his mother while still having an ownership interest in it.