Combine the strawberries and sugar into a very large saucepan.
Bring slowly to a full boil, and stir occasionally until the sugar dissolves.
Cook rapidly until it thickens, about 40 minutes.
As it thickens, stir it frequently to make sure it does not stick to the pan.
Pour into clean jars and place the lids on top (leave about 1/4 inch headspace so the jam can expand slightly). Give them a little bit of time to seal - you will hear a pop when they seal.
Notes
Boil the jars and lids for about 10 minutes to sterilize them. Make sure your saucepan is very large or your jam might boil over. Stir often during the 40 minutes or your jam might stick to the bottom.