Start with a line of redstone, leading up to a redstone lamp. Obviously, the restone AND the lamp should be powered, and they are. Then, place a block next to the redstone right up next to the lamp. Next, place redstone on top of that block. The redstone lamp should now turn off, and it does. Now, place another block on top of the redstone that was originally powering the lamp. Next, break the redstone that was placed on top of the block. Then, break both blocks. Now, you should be back to what you had at the beginning, which is just the line of redstone leading up to the redstone lamp. However, the redstone and the redstone lamp should BOTH be powered like it was in the beginning. But, the lamp is not! The redstone is, but after doing the above, the lamp is NOT on.