Most kitchens own twenty spices and use four, and the twenty are mostly old. Spices are the cheapest way to make the same ingredients taste like a different meal — but only if you buy the right ones, keep them properly, and treat them as something to be cooked rather than sprinkled.
The ten that earn their space
Black pepper, cumin, paprika or chilli, coriander seed, cinnamon, turmeric, dried oregano or thyme, bay leaves, mustard seed, and either ginger or garlic powder. With those you can cook convincingly across most of Europe, North Africa, the Middle East and South Asia. Everything beyond them is a specific dish you have decided to cook, not a gap in your cupboard.
Fry them, do not sprinkle them
Ground spices are mostly oil-soluble, which means they release their flavour into fat and not into water. Thirty seconds in the hot oil with the onions does more than a spoonful stirred into the finished pot. Whole seeds go in first, into the oil, until they pop and smell. Ground spices go in after the onions are soft and burn in under a minute, so keep them moving and add the liquid soon after.
They do not go off, they fade
Ground spices keep their character for about a year and then become dust that tastes vaguely of the cupboard. They are not dangerous, just pointless. Whole spices last three or four years and grind fresh whenever you want them. Test them by rubbing a pinch in your palm and smelling it. If you have to work to smell anything, the reason your cooking tastes flat is sitting in that jar.
Buy them in bags, not tiny jars
Supermarket spice jars can cost many times what the same spice costs by weight at a grocer or a wholefood shop. Buying properly is the cheapest upgrade in any kitchen, and it means you use them generously rather than measuring out half teaspoons. Buy less at a time, though. Half a jar of fragrant cumin beats a large bag of tired cumin, and the price advantage disappears the moment it sits for two years.
Store them away from the cooker
Heat, light and steam are what destroy them, so the rack directly above the hob — the place nearly everyone keeps them — is the worst spot in the kitchen. A cool dark cupboard doubles their useful life. Keep the lids on tight and never shake a jar over a steaming pan; the rising moisture gets into the jar and the spice clumps and stales. Tip into your hand first, then into the pot.
Ten good spices, bought in bags, kept in the dark, fried in oil rather than stirred in at the end, and replaced when they stop smelling. That is most of the difference between food that tastes homemade and food that tastes like nothing in particular.