And the rain came

 At last it is raining in Spain. 

To improve her diction Eliza Doolittle had to sing 'The rain in Spain stays mainly in the plain.'

Eliza Doolittle was the character in a play, written by Irish playwright George Bernard Shaw, called Pygmalon. It was later adapted to the cinema under the name 'My Fair Lady'. 

 The phrase is not true at all but good for practising the sound  /eɪ/