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How to check the freshness of baking powder
Boil some tap water.
Use tap water and fill a kettle to the minimum fill line. You only need cup (118 ml) of hot water to test the baking powder, but filling the kettle to the minimum fill line will ensure the heating element doesn't overheat.
Turn on the kettle and bring the water to a boil. Don't use more water than you need, otherwise the kettle will waste energy heating the extra water.