Mary Jane is stranded on a deserted island in the middle of a huge and deep lake. She cannot swim and all she has is two strings, each one one foot long. All of the boats she would be able to use are docked at the shore 4 miles away from her. 2 hours later, she is on the shore of the lake. How did she get across the lake?
This riddle has been copied from this page, where the answer is given as:
The lake was frozen so she was able to walk across, making it to the shore.
It probably is an example of a classic lateral thinking puzzle but to be honest I think it is very poor. There is no logic involved and ANY contrived "deus ex machina" answer could be substituted.
Nothing in the question either alludes to or excludes the possibility of the lake being frozen, but by the same token nothing excludes the possibility of Mary-Jane being rescued by a helicopter either. Further, Mary Jane is described as being "stranded" on the island, but a lake does not freeze in 2 hours so it must have been frozen right from the start in which case she was never really "stranded". Terrible puzzle.
I would guess:
that these boats are docked four miles away on the island.
which further suggests:
Mary Jane could walk over to the boats in enough time. According to Google, it takes the average person about 20 minutes to walk a mile. Thus, if she walked the entire time, she could potentially make it to the boats in an hour and 20 minutes. This leaves forty minutes if the boats are ready to go immediately.
This makes a few assumptions:
First, Mary Jane needs to know how to operate the boat. Second, they should be speedy boats to get offshore within the allotted time. Third, Mary Jane is able to traverse the terrain.
The most obvious answer, which I confess I hope is wrong, is that
she didn't get across the lake
after two hours she is on the shore of the lake on the island.
If this is right, then
the last sentence, for my taste, crosses the line between clever misdirection and outright deception, and I must direct the questioner's attention to this famous comic strip: https://xkcd.com/169/.
I remember hearing this puzzle a long time ago. I tried looking up the source, but I couldn't find it.
Anyways, the answer has been said by others, but they left out one crucial part: the strings we're not used in their answers.
The lake was frozen, and she had two shoelaces on her ice skates.