Learn Swedish: E-boken öppnar du med tummen. - The e-book, you open with your thumb.

In Swedish, it is standard to use the definite form for body parts when it is already obvious who they belong to. Since the subject doing the action is du (you), the listener automatically knows that tummen refers to your own thumb. Using din tumme here would sound unnaturally emphatic, as if you were clarifying that you use your thumb and not someone else's!

Is med always the right preposition to use when performing an action with a body part?

Yes! In Swedish, med acts as an instrumental preposition. Just as you use it to say you are doing something with a physical tool (like opening a box med en kniv - with a knife), you also use it for body parts that perform an action. You swipe med tummen (with your thumb), point med fingret (with your finger), or kick med foten (with your foot).

Why is the word order E-boken öppnar du instead of Du öppnar e-boken?

This is a very common Swedish sentence structure called topicalization. By putting the object (E-boken) at the very beginning of the sentence, the speaker places special focus on it. Because Swedish has a strict "V2" rule (the verb must be the second piece of information), the verb öppnar has to stay in the second position. This forces the subject (du) to slide to the third position, right after the verb.