Remembering to turn the gas off, or water the plants before a weekend away, can prove an impossible feat when the mind is churning with other tasks.

But Harvard University has uncovered a simple "brain hack" to bring back those forgotten to-do lists - the mental equivalent of tying a piece of string around the finger.

The trick is to associate the task with an object near to where it has to be carried out, which when spotted later on can trigger the missing memory.