There are a few elements that can cause knives to go blunt. Tomato and lemon juice are among the most common issues out there. They are quite acidic. The acid tends to corrode the blade. Using a knife daily for tomatoes and lemons is likely to corrode the edge within a couple of months. This is the main reason wherefore tomato knives are usually made of stainless steel. While still affected by corrosion, stainless steel is more efficient in front of it. Plus, such knives are usually ice-tempered to retain their edge in the long run.