At a screen you blink 7 times a minute, not 22

Time ten seconds on your phone and don't blink. If it starts stinging at six or seven, you now know the state of your tear film: in 1993, the ophthalmologists Kazuo Tsubota and Katsu Nakamori counted blinks in Japan and got 22 a minute at rest, 10 while reading on paper and 7 in front of a computer screen. Blinking is what spreads the tear film across the cornea, and at a screen you stop doing it without noticing.

Three things for tomorrow:

1. Drop the screen below eye level. Looking slightly downward, the eyelid covers more surface and the tear film evaporates more slowly.

2. Every twenty minutes, look at something six metres away for twenty seconds. That's the 20-20-20 rule, written by the optometrist Jeffrey Anshel, and it fits in a phone reminder.

3. When the stinging arrives, squeeze your eyes shut five times in a row. A half blink spreads nothing; the one that closes all the way does.

And if you work eight hours with the heating or the air conditioning on, leave a bottle of water in plain sight and drink every time you look at it. Tears are made out of what you drink.