US passports also need to be signed with a pen.

With the household teenager, this played out as might be expected before a trip to Hamburg: "You need to sign your passport." "Why?" "Because ze Germans will insist on it and won't let you in without it". After audible grunting and an eye-roll so dramatic I felt tempted to ask her to check her optic nerve for tangles afterwards, pen was applied to passport and our travels went undisturbed by German rulemongering.