When you see a rainbow, it means that someone is coming from Asgard, the land of the gods, to Midgard, the land of people, or else that someone is going home the other way. Everyone knows that. And when they get to the gate at Asgard, they'll find Heimdall, of course, guarding it. He is the best possible guardsman, because his eyesight and hearing are so good: anyone approaching is spotted while still a long way off. It is often said that no one gets past him who should not pass.

But it is sometimes said, and said much more quietly, that, on occasion, someone does. Any such someone must be very small, and very fast, and, very likely, our friend Heimdall must be busy admitting other travelers when they slip past.

So I want you to imagine now that this is one of those times. A few such interlopers have found their way into Asgard, into Thrudheim, the personal realm of Thor. And now it is night, and one of them wanders into Bilskirnir, the home of Thor and Sif, through a friendly open door. Thor and Sif are like that, you know.

They are asleep. Thor is dreaming of flying through the sky in his thunder chariot. But something is wrong in this dream. Instead of the fleeting wind, there is a pesky, high-pitched buzzing sound. Roused only slightly through the fog of sleep, it occurs to him that some tiny creature is making this noise. It is a mosquito. He has encountered these many times in visits to Midgard, and they are legion in Jotunheim. He bats it away and returns to his dream.

Now Thor is dreaming of battles with Frost Giants. One of them opens its mouth to roar an icy blast. But that is not what comes out. Thor is about to laugh at the Frost Giant with the tiny buzzing voice, but then the buzzing and the giant fade. It is the mosquito again. He bats it away a second time and goes back to sleep.

The mosquito does not even give Thor a chance to dream again before it is buzzing in his ear once more. He wakes a little more this time. Thor is annoyed. Thor thinks. It is that slow, ponderous, rumbling, creaking, heavy kind of thought that goes with being only half-awake.

Blarg! Gack! Mosquito won't go away.

But wait! Right over the edge of the bed, resting on the floor, is Thor's magic hammer, Mjolnir. It always hits whatever he throws it at. It was made to do that. That's what it does. He hardly even has to aim. This is what Thor is thinking.

Yes!

So, without even getting up, he reaches over, grasps his trusty iron friend by the handle and heaves it into the air with all his sleepy might.

The hammer knows its business. Before even leaving Thor's hand, it has scanned the room for its target. Mjolnir finds the mosquito over by the wall and heads for it at something over the speed of sound.

Wham!

And the mosquito is vaporized. So, of course, is much of the wall.

Sif, who was asleep beside her husband, is now no longer asleep. She sits up and sees a huge hole in the wall with starlight shining through. Meanwhile, the hammer is still in flight. It shrieks out there a few hundred yards, maybe half a mile or so, then turns around and starts back to Thor's hand. This also is what it does. Sif knows this and ducks just in time. Mjolnir lands back in its master's palm with a satisfying thwack! Thor sets it down again and is about to go back to sleep when Sif says:

"Oh, thank you, Thor! You started working on the window I've always wanted in that wall. You will finish it tomorrow, yes?"