Why a cup leaks and what to do about it

How the seal works

The cup holds not because it is shut tightly, but because two mechanisms work together. The cross-cut straw opens only when the child draws on it, and an air valve equalises the pressure inside so the drink is not pushed out.

If one of them is off, the other cannot compensate. That is why it pays to go through the parts one at a time.

Five things to check

Assembly. The silicone part must seat all the way round. If it is twisted or rippled even slightly, a gap appears.

The state of the silicone. Cracked or softened silicone does not seal. It is a consumable and is replaced on its own.

Drink temperature. A warm drink in a closed cup raises the pressure, which looks for a way out. Let it cool before closing.

Tightness. The cap must seat fully, but not so hard that the silicone deforms.

The straw. A cracked or badly fitted tube lets the drink out even when everything else is right.

When it still leaks

Take the cup fully apart, boil the silicone parts and let them dry. Then assemble it dry, fill it with water and turn it over the sink.

If it still leaks, one of the silicone parts is the culprit. They are sold separately and replacing one is cheaper than a new cup.

What is normal and what is not

A few drops around the straw after the child has thrown the cup on the floor is normal. So are a few drops after a sharp change in temperature.

Steady leaking from a cup standing still is not — then a part is not seated.