If you don't have Greek yoghurt on hand, you have a few options for making tzatziki: 1. Strain Regular Yoghurt: Line a fine-mesh sieve or colander with cheesecloth or a clean kitchen towel. Place the yoghurt in the lined sieve and set it over a bowl to catch the whey. Let it drain in the refrigerator for a few hours or overnight until it reaches a thick, creamy consistency. 2. Sour Cream: It has a slightly tangier flavour than Greek yoghurt, so you may want to adjust the amount of vinegar or lemon juice in the recipe. Be sure to use full-fat sour cream for the best texture. 3. Labneh: This Middle Eastern strained yoghurt cheese is even thicker than Greek yoghurt and has a slightly tangier flavour. It works well in tzatziki, but you may need to thin it out slightly with a bit of water or milk. 4. Vegan Alternatives: For a dairy-free option, you can use thick coconut yoghurt or a cashew-based yoghurt alternative. Keep in mind that these will have a slightly different flavour profile than traditional tzatziki.