Boiling water. whenever I try to boil a cup of water (in an 8 oz glass measuring cup) it boils for

Question

whenever I try to boil a cup of water (in an 8 oz glass measuring cup) it boils for a,second or two and then it violently expodes (all the water converts to steam and explodes out of the cup leaving most of the water in the bottom of the microwave.) what am I doing wrong? This is a 1000 watt microwave.