A scholar needs to cross a swiftly flowing river.
He is pleased to see that there is a ferry. He climbs on board and he and the ferryman set off into the current.
'How long will the crossing take?' enquires the scholar.
'Depends,' replies the ferryman. 'With the current as strong as what it is, it will take a while.'
'Did you never study grammar?' asks the scholar. The ferryman replies in the negative.
'Then you have wasted half your life,' snorts the scholar.
A few minutes later the ferry is driven onto rocks by the fierce currents.
'Did you ever learn to swim?' asks the ferryman.
'No,' replies the scholar.
'Then you have wasted all yours because we are sinking.'
Pedant at large, copy-editor for hire