Most houseplants that die indoors don't die of thirst. They die of kindness: a splash every few days, a pot that never quite dries, and roots that slowly run out of air. The catch is that a drowning plant and a thirsty plant can look the same, with yellowing leaves and a droopy posture, so the natural reaction to a sad plant is to give it more water.
Here is why too much water does harm, how to tell when a pot really needs a drink, and a small pot you can pour water into to see it happen.
Soil is not a sponge full of water. It is a mix of crumbs and air gaps, and roots use both. When a pot stays saturated, the gaps fill up, the soil runs short of oxygen, and roots start to rot. Gardening advisers such as the Missouri Botanical Garden and the RHS both make the same point: overwatering can lead to low oxygen in the soil and root rot, and water left standing in a saucer can drown the roots.
Underwatering also harms plants, and both problems can show up as yellow or wilting leaves, so the leaves alone won't tell you which one you have. The soil will.
The RHS advice is simple: don't water on a routine, check first that the pot needs it. Three quick checks:
This is a cross-section of a pot with 20 cm of soil. Pour some water, then let days pass. The finger on the right is draggable: pull it down the side of the pot to feel the soil at any depth. Try both pots, try pouring a little every day, and try a cool room against a warm one.
Try this sequence. With the drainage-hole pot, pour 8 and watch: the soil soaks through, the extra runs out, and over the next few days the top dries first while the bottom stays damp. Wait until the finger reads dry, pour again, and that is a healthy cycle. Now switch to the pot with no hole and do what a worried plant owner does: pour 4, press "Next day", and repeat for a week. The soil fills up, the extra has nowhere to go and sits at the bottom where the roots are, and the roots note turns from green to terracotta even though the top of the pot may look and feel fine.
Then compare rooms. In a cool room the same pot dries more slowly, which matches the RHS note that plants in cooler rooms need less water than ones in warm rooms. Central heating dries air and compost, but a plant on a cold windowsill still drinks slowly.
The finger test is a good default, not a law. Succulents and cacti are usually happier with the whole pot drying out between waterings, while some leafy plants prefer to stay evenly slightly damp. Check what your plant likes, and if it is a gift with no label, start by watering a little less often than you think and see how it responds. A wilting plant in damp soil needs less water, not more, so always check the soil before you pick up the jug.
If you suspect rotted roots, take the plant out and look. Healthy roots are usually firm and pale; rotted ones are soft, dark and may smell. You can trim the soft ones and repot into dry, fresh mix in a pot with a hole, but this is a last resort. Prevention is easier.
Pick a day each week to check your plants rather than to water them. Lift each pot, do the finger test, and water only the ones that need it. It takes a few minutes, it costs nothing, and it is the single habit most likely to keep your plants alive.