מבטל כלי מהיכנו (nullifying a vessel from its normal usage). Putting a כלי (utensil/vessel) under candles to catch oil, to catch the candle and...
a tray on a high chair; wheels on the bottom of cribs; wheel of a carriage;
הכנה ליום אחר (Preparations for Weekday): Understanding What It's All About;