It always frustrates me when I do a caulking or sealant job around the house to have to throw away a half used tube of caulk or silicon sealant or construction adhesive. I would use the provided caps and/or tape the tip, however no matter what I did the tube would be bad by the next job.
Here is a simple technique that will extend the life of a half used tube.
Step 1: What You Need
Here is what you need:
1. A circle of material cut from a milk jug.
2. A lighter or candle.
Make sure the nozzle or tip of the tube is clean of caulk.
Step 2: Use the Lighter
Use the lighter to heat the center of the material that you cut from the milk jug. Move the heater in the circular motion until you see the material turn transparent. The transparent area should be about 1 1/2 to 2 1/2 inches in diameter.
Step 3: Seal the End of the Tube
When you have the material transparent, quickly push the material down over the tip. The softened material will stretch forming a molded cap. If the material of the tube's nozzle is polypropylene (same material as the milk jug), the materials will sometimes bond. If you materials are not the same, you will just have a tight fitting cap.
Step 4: Trim
After everything is cool, trim the new cap you have made. If you want you can even add tape around the base of the new cap.
This technique will not keep your half used tube fresh forever, but it will extend the time that it takes the tube to go bad by a long time.