We can conduct a small experiment: put some books in our backpacks, about 2 kilograms will suffice. First, grab the backpack straps with our hands and hold the backpack in the air without moving it for a minute or two to feel the sensation; then tie a sturdy thin string (the thinner the string, the better) to the backpack, and hold the string with our hands to keep the backpack in the air for another minute or two. We will find that our hands will feel very sore when holding the string compared to holding the backpack straps. In the above two situations, although we are holding the same weight object and the force exerted on our hands is the same, our sensations are different. The reason for this is that our sensations (or the effects of the force) are not only related to the size of the force, but also to the area of contact with the force. In physics, we use the concept of "pressure" to describe this effect. Simply put, pressure is equal to force divided by the area of contact. When lifting the backpack, although the force exerted on our hands is the same, the area of contact with the string is smaller than that with the backpack straps, so the pressure is greater and the effects of the force are stronger, which is why it feels very sore to hold the string.
If we observe a thumbtack, we will find that the head of the thumbtack that comes into contact with our hand (the tack head) has a relatively large area, while the tip of the thumbtack that comes into contact with the wooden board has a very small area. When pressing the thumbtack with our hand, the force exerted on our hand and the force exerted on the wooden board are the same, but because the area of contact between the tack head and our hand is hundreds of times larger than the area of contact between the tack and the wooden board, from the perspective of the effects of the force, the pressure felt by the wooden board is hundreds of times greater than that felt by our hand. Therefore, we can easily press the thumbtack into the wooden board with our hands.