A sifter is a device designed to get any lumps out of powders like flour for cooking, or to mix different powders together. A sifter is very similar to a sieve but it has a handle on the side that turns some blades inside the sifter around to make the process of sifting the powders faster and easier.
A sign is something that conveys some kind of meaning. Signs might be made up of pictures, words, or a combination of both. Many signs are just symbols, so that people will recognize what they mean no matter what language they speak. Signs don't need to be painted on a board - for example, deaf people communicate with sign language, using their hands to make symbols.
Silence is the state of there being no noise. When it is very quiet you might say there is silence.
You may also use silence in terms of nobody talking about something for some time. For example "The celebrity is back in the media after a time of silence".