WHY IS ONE OF OUR TOADS STAYING ON THE BACK OF THE BIG TOAD?

  we have two toads, one big one and a smaller one, the small one has been staying on the back of the bigger one for about a week now. Why is this happening
The small toad on the the back is the male, they are often smaller and different in color. The big one on bottom is a female. They are mating. They will stay like this until they are done, do not seperate them as it could hurt them.
Naomi's right, they ' re mating, they'll separate when they're ready to, just let it take it's natural course. and please do NOT attempt to separate them, as she said it can injure them greatly.
They're probably screwing.