The image you sent is a question asking why some foods taste sweet and others sour. The answer is that different foods contain different chemicals that trigger different taste receptors on our tongue.
Sweet foods contain sugars, which bind to taste receptors called T1R2 and T1R3. These receptors send signals to the brain, which interprets them as sweetness.
Sour foods contain acids, which bind to taste receptors called T2R3. These receptors send signals to the brain, which interprets them as sourness.
There are other taste receptors that detect other flavors, such as salty, bitter, and umami.
The intensity of the taste we perceive depends on the concentration of the chemicals in the food and the sensitivity of our taste receptors.
Our sense of taste is also influenced by our expectations, our mood, and our overall health. For example, if we are expecting a food to be sweet, we are more likely to perceive it as sweet, even if it is not very sweet.
So, the reason why some foods taste sweet and others sour is because they contain different chemicals that bind to different taste receptors on our tongue. The intensity of the taste we perceive depends on the concentration of the chemicals in the food and the sensitivity of our taste receptors.