Most of the times, we use only 3/4 of the total length of a Pencil, because it inconvenient to write and sharpen. The remaining portion is either thrown away or lost. The manufacturers can make the pencil only 3/4 of its current length and attach a 1/4 length dummy extension at the end. This extension
can house an eraser.

The pencil extender is an extension to the normal pencil. The total lengh of the pencil with extender is longer than the regular pencil. In my idea, the extender will be a part of the pencil in its standard length. So people can use the 3/4 the length and throw away the remaining portion .