Here's the rundown of the elements and how they interact. 

TUBE:
TUBE a hard outer shell and the inside immediately transforms into whatever element is placed into it, very useful.  

SPAWN:
SPAWN immediately turns into a creator of whatever element is placed upon it.  This works for VAPOR also
Exceptions:
GUNPOWDER turns SPAWN into HEAVYFIRE
WALL turns SPAWN into TIMERWALL
PLANT turns SPAWN into WOOD
Unless otherwise stated, assume an element is reactive with SPAWN 

TIMERWALL:
Used as a timed release, TIMERWALL is a bright yellow wall that sits nonreactive until met with SALTWATER which then at the same time, the entire TIMERWALL slowly disappears. This innovative element is heavily useful.  

WALL:
WALL is a classic and generally nonreactive except against ACID, which eats right through it. WALL is useful for containing and directing elements. 

GLASS:
GLASS, like wall, if relatively nonreactive but is ate through by SAND
Nonreactive to SPAWN

FUSE:
FUSE is like glass and wall but when in contact with fire, burns. 
Nonreactive with SPAWN

PLANT:
PLANT is a classic and absorbs water and is also burnable. Very useful. When in contact with SALTWATER, the outer layer of the PLANT dies and turns into WOOD

WOOD:
WOOD is created by PLANT + SPAWN or SALTWATER + PLANT. Burnable, WOOD reacts with WATER by producing SEED.

SEED:
SEED, although flammable, is not highly reactive with FIRE. SEED falls slowly and turns into plant when in contact with water, very useful.

SAND:
Heavier than WATER, SAND is useful for boring through GLASS and for raising water level. 

SALT:
SALT is a classic and when in combination with WATER, SALTWATER is formed. 

WATER:
A classic and key element, WATER is used for many things. WATER is taken over by PLANT or SEED, is turned into WATERVAPOR by FIRE, eats through HEAVYFIRE, and can put out FIRE. 

WATERVAPOR:
Put off when WATER and FIRE intermingle, WATERVAPOR rises as a gas but then condenses and falls as a liquid. Very useful and fun to plan for and play with.  

C4:
C4 is a classic heavily explosive wall material. 

ACID:
Considered 'heavy', ACID sinks through most other liquids and eats through WALL

OIL:
OIL is  a classic liquid that is rather flammable and floats on water. 

FIRE:
FIRE is a keystone classic for most 'games' played. FIRE turns WATER into WATERVAPOR and sets many things on fire such as: FUSE, PLANT, C4, OIL, NITRO, BUTANE, GUNPOWDER. 

HEAVYFIRE:
Created by the mixture of SPAWN and GUNPOWDER or SPAWN and C4, HEAVYFIRE  is a wall material that drops FIRE onto whatever is below it.

NITRO:
NITRO is a very explosive classic element that explodes in a firework fashion, makes for a very good ending element. 

BUTANE:
BUTANE is one of the few gasses in the game. Not very flammable on its own, when in large quantities BUTANE creates quite an explosion. 

GUNPOWDER:
A classic, gunpowder is rather flammable and drops like a liquid.