I invented this when i was bored at school and accidentally spilled something on my eraser.
Step 1: Dirty Eraser
First get your dirty eraser, like this one that was dirtied by a pencil of mine.
Step 2: The Secret Weapon, Glue!
Next, get any type of glue, but i recommend using this stick glue or liquid glue, but in the end they both will work.
Step 3: Apply the Glue
The third step is to apply the glue on your eraser, apply it thrice so it's nice and sticky.
Step 4: RUB IT IN!
Now for the hard part, use your thumb, and using the rest of your fingers to hold the eraser, hardly rub the glue all around the dirty eraser like in the photo. It is alright if your thumb gets super sticky, you can wash it later after cleaning the eraser.
Step 5: Halfway Done! :)
Look! We are almost done! Continue to rub the eraser with as much force as you can!
Step 6: Super Clean Eraser
Once you are done, you would see your eraser as clean as the one in the photo. If it is not, continue rubbing hardly. But don't forget that once you clean your eraser, you will have to wash it with water to get rid of the stickiness. If you find another thing this technique can work on, please tell! I hope you find my technique useful! :D