Is it safe to put a candle in sand?

Yes, but the sand might not support the candle very well, and if the lighted candle were to fall over, there might be a fire danger depending upon where it is located and what materials, other than the sand, surround it. Putting a candle in sand at the beach, for example, would be no problem at all even if the lighted candle were to fall over. But putting a candle in a bowl of sand on the dining room table would be risky because if the lighted candle were to fall over it might cause a fire. Of course, if you don't intend to light the candle, putting it in sand is perfectly safe.