In a medium saucepan over medium heat, add the berries and sugar.
Mash the berries with a potato masher to get some of the juice going, and then bring to a simmer.
Cook the jam until it thickens and starts to cling to the bottom of the pan. Stir it ever 2-3 minutes, and keep an eye on it, about 10 minutes.
Once it threatens to stick to the bottom of the pan, stir it constantly for 1 minute to thicken further.
Pour into a pint jar (or two half-pint jars). Let cool to room temperature. Store in the fridge. Use within 2 months.
