Final answer:
Keratinization due to sucking or resting between teeth is caused by both actions, as they can lead to chronic irritation or pressure in the mouth, prompting the oral mucosa to produce more keratin.
Step-by-step explanation:
The cause of keratinization due to sucking or resting between teeth is both sucking and resting between teeth. Keratinization is a process where the outer layer of the skin, or in this case, the oral mucosa, becomes thicker and infused with keratin, a protective protein. This can be a response to chronic irritation or pressure in the mouth, which can result from habits like sucking on objects (like thumbs, pacifiers, or pens) and resting the tongue or other objects between the teeth. These actions can cause the cells in the mucosa to produce more keratin as a defensive mechanism. It is important to maintain good oral hygiene and minimize habits that can lead to keratinization as it can sometimes interfere with normal oral function or become cosmetically displeasing.