If you confirm others' transactions, what do you do to earn 25 bitcoins per block? I was a bit surprised to learn that you can earn 25 bitcoins, just by confirming a block worth of transactions. If I've done the math right, at $493 per bitcoin, that's $12,325.

Does one do a lot of work (beyond a few mouse clicks) to earn those 25 bitcoins? Does that constitute "rent" for the use of the resources of your computer (which probably cost a lot less than $12,325)?

Or is it just the case that 25 bitcoins are issued per block, and by confirming  a block of transactions, you get those those 25 bitcoins.
Just to give some specifics in addition to David Grayson's answer:

At current difficulty levels, mining a block requires computing an average of


  29,973,900,941,385,007,104


hash operations.  An ordinary computer can perform perhaps 10,000,000 such operations per second.  At that rate, such a computer would take about 95,046 years to mine one block, on average.  If the computer draws 100 watts of electricity, it would use 3,469,201 kilowatt-hours of electricity over that time.  Electrity prices are somewhere around US $0.10 per kWh, so that would cost US $346,920 in electricity alone - much more than the $12,325 of the block reward.

For obvious reasons, most miners today don't use ordinary computers, but rather specialized ASIC hardware that is considerably more efficient (and more expensive) than an ordinary computer.