A sure cure for seasickness is to sit under a tree.

Milligan’s line is a characteristically deadpan, absurdist “solution” that works by sidestepping the problem rather than treating it: if you’re under a tree, you’re not at sea. The humor comes from the mock-medical phrasing (“a sure cure”) paired with an impractical, literal-minded remedy. It also gently satirizes the public appetite for quick fixes and authoritative advice, exposing how easily “cures” can be made to sound convincing when they redefine the conditions. As with much of Milligan’s comedy, the joke rewards logical thinking taken to a ridiculous extreme.