With the earth quake in Japan that has set off a tsunami waring for the entire west cost of North and South America and Hawaii there may be some students that have questions. Below are some pages will you explain what a tsunami is. The pages would also work well for an Earth Science or weather lesson.
FEMA for KIDS - Tsunami
NOAA - Tsunami for Kids - This page has links to several activities including games, coloring pages, and a video. The video is very good and gives a lot of good information. I know more about tsunamis after watching it.
Tsunami - Derby Ridge Elementary Columbia, MO - This page also has several kinks to more information about Tsunamis.