Since caffeine is soluble at about 2 grams per 100ml of room-temperature water, or 2.2mg per each milliliter, the most dissolved caffeine that 8mL of room-temp water can hold would be 18mg; at a 1 ...

Caffeine is very soluble in boiling water, so there is some basis for this idea, although the second brew may be very lacking in flavour. If you really want to decaffeinate a teabag, you might do ...