A father promised his son that they would go for a walk if the son had completed his homework before tea-time. When asked later, the son said he had completed his homework (even though he hadn't). So off they went for a walk.
After a while they came up to a bridge. "That is a liar bridge" said the father "it collapses when a liar walks across it".
This scared the son who confessed about his homework. They started on to the bridge which immediately collapsed - because there is no such thing as a liar bridge.
