Dull knife is not good tool to cut and you need blade sharpening to make it sharp again. When discussing about sharpening tool, there are several things to understand. This is tool with capability to turn the dull edge on blade to be sharp. In general, you just reduce the edge angle into more precise and lower as keen as possible. In order to find proper tool for sharpening, you should check the compatibility between blade and its sharpening tool.
In past time, sharpening tool was manual and straightforward. There were two methods for turning dull edge into sharper. Firstly, you used whetstone to rub on the blade surface, particularly at the edge area. Be careful when doing this process and make sure to rub only one way. You did forward or backward depending on blade position. This type of process required whetstone to move, blade stood still without motion. On the other side, the second method was you did opposite way. In this sharpening process, you attached and moved blade at the top of whetstone. Some people called this process as grinding, but manually. Both are the old ways for blade sharpening.
Today, several manufacturers produce various tools for sharpening knife. Keep in mind that blade in this term is what you use to cut in small form. Of course, there are many things that considered as blade, but the size is bigger than normal knife. Even kitchen blade is still considered as knife that needs to be sharpened. Blade sharpening process has one purpose, which is sharp edge for cutting anything based on blade purpose.
For pocketknife, you can have sharpening tool next to its handle. This kind of knife is attached directly with its tool for sharpening. You do not have to worry when the blade is dull because sharpening process will be easy and simple anytime. Another process is similar to automatic grinding to reduce tiny amount of blade. This kind of tool has feature to measure sharp area to keep the balance for entire edge. When using manual tool, you only rub and swipe it until the blade is sharp enough for slashing. Unfortunately, you cannot keep the edge measurement at precise level.
After sharpening process, the last step is to test whether the blade is sharp enough to do its task or not. You can check directly on its edge, but it requires keen eyes. The old way is to touch its edge, but be careful. Thirdly, you just cut into what it supposed to be. From those ways, you can verify that blade sharpening tool is working properly.