This four letter word can be an animal, but you might also use a wax version of me to close a letter. I can sometimes be airtight, or you might even talk about a lab being hermetically ____ed

In metal skin they toil and trot, with gears that never tire or rot. Like a clock's ticking heart, a helping hand that's hard to stop.

I am an armoured army of thousands, standing on a single leg and blindly chasing a golden god. When youth leaves me, my neck stiffens into a permanent morning prayer, and my heavy head bows to yield a bounty of black-and-white shields. What am I?

I was built to fly, yet someone insists on holding me back.

I can be hidden, a national bank, or a girl - like totally. But you probably know me best by my adjacent association with mountains and hills.