A sieve is a mesh strainer used to separate lumps and clumps from the fine material. Sieves are handy for everything from sifting flour to prospecting for gold — anything where you need to separate the big from the small.
If you’re an archaeologist, you've probably used various sorts of sifters and shakers to sieve through the soil to recover even the tiniest artifacts. If your interests run more to building sandcastles, you’ve probably got a sieve in your beach bag, along with the shovels and pails. If you keep forgetting things, you can describe your "mind as a sieve," since it doesn’t seem to hold much.