Most home repair seals and caulks are opened by cutting away the tip. We're not sure why, but it's probably a strategy to get you to buy a new tube every time you need to make some simple repairs. However, you can actually use duct tape to seal the tubes up and save them for the next time you need them. Simply wrap the area you cut away and store them wherever you store your tools.
If you want to be extra fancy, you can cut a small hole in the duct tape and hang your tubes as shown in this photo, but that's totally optional.