STAD aluminium pen caps, I supposed that is how it is known as.
I got introduced to these recently, and decided to buy a pack to try it out.
Each pack consists of 4 pencil caps, and it comes with 4 labeling stickers - It will be useful if you are buying this for your children.
Because of the soft material used, some might suffered a dent during the packing process;
but the dent can be easily pushed back to shape.
Capping the pencil is to prevent unnecessary pen marks make on the other stationeries while storing the pencil inside the pencil case.
Uncapping it, acts as an extension to the short pencil.
I am using this to cap my "pencil refill" for my STAD One Push Pencil Holder, so as to prevent the pencil lead from breaking through rough handling of my pencil case.
There are 3 different packings available, and you might want to explore your local stationery stores for this little useful caps.