«Mysterious Tadpole, The» by Steven Kellogg

It's Louis's birthday and Uncle McAllister has brought him a very special gift - a tadpole all the way from Scotland! Louis can hardly wait for Alphonse to grow into a frog. But it soon becomes clear that Alphonse is not turning into any ordinary frog. First Alphonse outgrows his jar, then the sink, and then the bathtub!