Phaeton son of Helios was not able to control the sun god's chariot when he took the reins without permission - scorching the earth. Zeus hurled a thunderbolt - destroying the chariot and killing Phaeton whose body fell into the sea. His sisters gathered at the water and wept for their brother. Zeus, unable to bear their lamentation turned them into poplar trees ringing the shore. 

Even as trees they continued to weep. Their tears were transformed into amber.