1. In a large skillet or saucepan, mash up raspberries to desired consistency. Turn heat on high, then add sugar and stir continuously until mixture comes to a boil. Add lemon juice, and keep stirring.
2. To test if the jam is done, put a spoonful of jam into a cold bowl and then taste. If right consistency and taste, turn off heat and jar in a sterile jar immediately. If not, keep stirring and rechecking.
3. Allow to cool before serving.