This is a classic prank. It is fast and easy, and only requires packaging tape, a side sprayer, and a victim.
Step 1: Tape Down the Button
Take a towel, and wipe down the handle of the sprayer to get rid of any moisture that would cause the tape to not stick properly.
Now simply take a piece of packaging tape(It doesn't need to be too long. A couple of inches should do it), and wrap it around the button on the side sprayer. Wrap it tightly so the button won't pop back up.
Make sure the smooth out the wrinkles on the front of the sprayer where the victim might see it.
Step 2: Test It Out
Turn on the water, and make sure the water comes out of the side sprayer properly.
Now just put the side sprayer back, and make sure it's facing forward.
Now just wait for an innocent victim to come along, just wanting to wash their hands...
WARNING: Mothers usually don't appreciate this prank as much as other people...