Barking Mad by Tom E. Moffatt

At first, Fingers refuses to believe that his Granddad has gone BARKING MAD!

But what straight-thinking grownup goes around LICKING the postman, growling like a dog and chasing hospital security guards up trees?

And when Fingers and his sister Sally discover a BIZARRE machine in Granddad's workshop, mix-ups turn into MIND-SWAPPING madness - one look at Granddad's dog DaVinci is proof of that!