Time is a great topic for project work at KS3. Set your students to design and make an instrument to measure one minute. Judge the entries on three counts, accuracy, most original design, most manageable and workable on a roller coaster. An alternative project would be to set up a Rough Science scenario where the students need to design a way to tell the time from what you would find on a desert island. Human sundials are also great fun to make up on a playground.
At a more advanced level students might question how the second is measured to be so. The caesium atom radiates radiation of frequency 9,192, 631,770Hz from electron transitions when stimulated, and since frequency is a measure of the number of oscillations hence the definition. There is more detail on how this standard is arrived at on the website of the State University of New York Institute of Technology at http:sts.sunyit.edutimetechstateofart.html