I was wondering this the other day. Is there an easy way to create a perfectly flat surface (or a ruler-straight line) without any tools?
…and no, I don’t need an answer urgently. 
Plumb line with a palm frond and a coconut gets you a vertical.
Horizontal level is most easily measured with a trough of liquid- dig a ditch and let water seep in.
Yes, I can build a hut from bamboo. And build fire. Please don’t vote me off the island.
Make a rough form and put some type of homemade concrete out of it based on crushed seashells and sand. If the consistency is right, it will naturally become a flat surface when it is wet and then harden.
If you want solid flat surfaces, get three rocks, as close to flat as you can find. Rub rocks 1 and 2 against each other, then 2 and 3, then 1 and 3, and repeat. With enough rubbing, they’ll wear down to flat.
With just two rocks, you’ll generally end up with spherical surfaces on both, one concave and one convex. But this can’t happen with three.
Pour water into any container.
That’s the first thing I thought of. The problem is, it wouldn’t make a very good work surface or ruler.
Sit-ups.
[Pedant Mode] As the Earth is a sphere, no you can’t get a perfectly flat surface. Parallel lines may be out as well. [/Pedant Mode]
You haven’t told us what you are doing. Why do you need a flat work surface? What are you measuring, and why do you need a ruler?
Have the Professor work it out; keep Gilligan otherwise occupied.
Make a spirit bevel the way the Egyptians did to make sure their pyramids had a perfectly level base: dig a ditch around the “footprint” of the building, or in your case, just a square area on the ground. Fill the ditch with water, and then “grade” the ground inside the square according to it’s height above the water.
Just don’t know how you’re going to dig a ditch and carry water without tools.
I use coconuts, palm wood, water, and sand to make a coco-robo contractor.
Build a bamboo sand box as level as you can. Fill with sand. Pull a string taut and drag across surface to smooth.
There isn’t any easy way to create a *perfectly *straight line with the best tools in the world. Any solid object is going to deviate from truly straight by some margin.
The next issue is what tools you are allowing us to make. Do you literally mean “no tools”, or are we allowed to use things like stones, fire and water?
You can get a good approximation of straight simply by folding a thin object in half. A segment of leaf with no major veins will produce a straight edge. If you want to spend the time pulverising grass, soaking and drying it, you can make a thin sheet of paper that can be folded into an edge as straight as, well, a thin piece of paper.
Even if your island has no bees, you can extract resin from a great many plants by scraping the wax off the leaves. Then all you need to do is float the resin across the surface of a pool of warm water. Even if no buckets are available, you can find a rock pool and keep dropping hot stones into it until it reaches the desired temperature. As the water cools, the resin will solidify, and you can pick up a perfectly flat resin block.
There are any number of ways to achieve an arbitrarily flat/straight surface. You really need to tell us what you want to use it for.
Basically I was running under the assumption that having access to a flat surface and something to use as a ruler would be a prerequisite to getting anything else built.
Love this!
Must add that the orientation of the rocks in question needs to be changed continually during grinding. That is, one is rotated with respect to the other on the axis perpendicular to the surface being ground. Otherwise two rocks are as likely to form a cylinder or ellipsoid or even a saddle.
I guess that it would be good practice to do the same with the three rocks. Although my brain is currently doing flips trying to determine the effect of different grinding modes.
Not really. You would want a way to measure one thing relative to another, but you don’t need a length. And you’d put together a couple of sawhorses to hold your materials, not use a table anyway. A vertical line can be found with a plumb bob.
Cook and eat Gilligan. Immediately.
In the real world, beaches have junk washed up on them. It’s completely possible you could find a piece of plywood, or something like a wire spool.
That would give you an instant table, or possibly even a cart.
shakes fist in general direction of edit window
For desert island purposes, your arm is a ruler. You can measure things Egyptian style, with cubits, hands, spans and fingers.
Probably this.