Give some stream of consciousness and then the answer. The shrine buildings at Naiku and Geku, as well as the Uji Bridge, are rebuilt every 20 years as a part of the Shinto belief of the death and renewal of nature and the impermanence of all things and as a way of passing building techniques from one generation to the next. The twenty year renewal process is called the Sengu. In August, in a long-standing tradition, the people who live in Ise are allowed to enter the area around the Inner Sanctum of the Naiku as well as the Geku. Some villages drag a wooden carriage laden with white stones up the Isuzu River onto the grounds of the Naiku. Each participant gets two white stones in a white handkerchief and these allow them to place the stones in the area around the Inner Sanctum. Other villages drag a huge wooden car or Noburi Kuruma laden with white stones to the Uji bridge at the entrance of the grounds of the Naiku. Participants receive two white stones which are also placed in the sacred space around the Inner Sanctum. The entire tradition is called Shiraisshiki and it is very colourful with every participant wearing a' happi' coat representing a particular village. The rebuilding of the main shrine takes place on a site adjacent to the old, and each rebuilding alternates between the two sites. The next scheduled rebuilding of Naiku is due in 2033 on the lower, northern site.

Answer this question: why is the naiku (inner) shrine at ise rebuilt every twenty years?
This is the relevant information: The shrine buildings at Naiku and Geku, as well as the Uji Bridge, are rebuilt every 20 years as a part of the Shinto belief of the death and renewal of nature and the impermanence of all things and as a way of passing building techniques from one generation to the next.
Therefore, the answer is as a part of the Shinto belief of the death and renewal of nature and the impermanence of all things and as a way of passing building techniques from one generation to the next.