Do like an otter, add acid to water.

Mnemonic Device:

• Do like an otter, add acid to water.
• Do what you Oughter, add acid to water!
• Acid to Water, All is Well. Water to Acid, What an Accident!
• If you’re doing what you oughter,
  Add the acid to to the water.
  May your rest be long and placid,
  Adding water to the acid.

Explanation: To remember the order, for chemistry, when combining acid and water. That it is OK to add acid to water. But you may get an explosion, from the excess heat created by reversing the order, when mixing the two.